/* ===== RESET & BASE ===== */
*,
*::before,
*::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root {
    --bg: #0a0a1a;
    --bg-card: #12122a;
    --bg-card-hover: #1a1a3a;
    --text: #e8e8f0;
    --text-muted: #8888aa;
    --text-dim: #555577;
    --blue: #4d8ef7;
    --blue-glow: #4d8ef730;
    --purple: #9b59f5;
    --purple-glow: #9b59f530;
    --teal: #2dd4bf;
    --teal-glow: #2dd4bf30;
    --emerald: #34d399;
    --rose: #f472b6;
    --rose-glow: #f472b630;
    --orange: #fb923c;
    --amber: #fbbf24;
    --border: #1e1e3a;
    --radius: 16px;
    --radius-sm: 10px;
}

html {
    scroll-behavior: smooth;
    scroll-padding-top: 80px;
}

body {
    font-family: 'Inter', sans-serif;
    background: var(--bg);
    color: var(--text);
    line-height: 1.7;
    overflow-x: hidden;
}

code {
    font-family: 'JetBrains Mono', monospace;
    background: #1a1a3a;
    padding: 2px 8px;
    border-radius: 6px;
    font-size: 0.95em;
    color: var(--teal);
}

strong {
    color: #fff;
}

a {
    color: var(--blue);
    text-decoration: none;
}

/* ===== ANIMATED BG ===== */
.bg-grid {
    position: fixed;
    inset: 0;
    z-index: -2;
    background-image: radial-gradient(circle, #ffffff06 1px, transparent 1px);
    background-size: 40px 40px;
}

.bg-glow {
    position: fixed;
    z-index: -1;
    border-radius: 50%;
    filter: blur(120px);
    opacity: 0.3;
    pointer-events: none;
}

.bg-glow-1 {
    width: 500px;
    height: 500px;
    background: var(--blue);
    top: -100px;
    left: -100px;
    animation: float1 15s ease-in-out infinite;
}

.bg-glow-2 {
    width: 400px;
    height: 400px;
    background: var(--purple);
    bottom: 0;
    right: -50px;
    animation: float2 18s ease-in-out infinite;
}

.bg-glow-3 {
    width: 300px;
    height: 300px;
    background: var(--teal);
    top: 50%;
    left: 50%;
    animation: float3 12s ease-in-out infinite;
}

@keyframes float1 {

    0%,
    100% {
        transform: translate(0, 0)
    }

    50% {
        transform: translate(80px, 60px)
    }
}

@keyframes float2 {

    0%,
    100% {
        transform: translate(0, 0)
    }

    50% {
        transform: translate(-60px, -80px)
    }
}

@keyframes float3 {

    0%,
    100% {
        transform: translate(-50%, -50%)
    }

    50% {
        transform: translate(-30%, -30%)
    }
}

/* ===== NAVBAR ===== */
.navbar {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    z-index: 100;
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 12px 32px;
    background: #0a0a1acc;
    backdrop-filter: blur(20px);
    border-bottom: 1px solid var(--border);
    transition: box-shadow .3s;
}

.navbar.scrolled {
    box-shadow: 0 4px 30px #00000060;
}

.nav-brand {
    display: flex;
    align-items: center;
    gap: 10px;
}

.nav-icon {
    font-size: 1.5rem;
}

.nav-title {
    font-weight: 700;
    font-size: 1.1rem;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

.nav-links {
    display: flex;
    gap: 8px;
}

.nav-link {
    padding: 8px 18px;
    border-radius: 99px;
    font-size: 0.85rem;
    font-weight: 500;
    color: var(--text-muted);
    transition: all .3s;
    border: 1px solid transparent;
}

.nav-link:hover,
.nav-link.active {
    color: #fff;
    background: #ffffff10;
    border-color: #ffffff15;
}

.nav-progress {
    display: flex;
    align-items: center;
    gap: 10px;
}

.progress-bar {
    width: 100px;
    height: 6px;
    background: #1e1e3a;
    border-radius: 99px;
    overflow: hidden;
}

.progress-fill {
    height: 100%;
    width: 0;
    background: linear-gradient(90deg, var(--blue), var(--purple));
    border-radius: 99px;
    transition: width .5s;
}

.progress-text {
    font-size: 0.8rem;
    color: var(--text-muted);
    font-weight: 600;
    min-width: 32px;
}

/* ===== HERO ===== */
.hero {
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 120px 60px 60px;
    gap: 40px;
    position: relative;
}

.hero-content {
    max-width: 600px;
}

.hero-badge {
    display: inline-block;
    padding: 6px 16px;
    background: linear-gradient(135deg, var(--blue-glow), var(--purple-glow));
    border: 1px solid #ffffff15;
    border-radius: 99px;
    font-size: 0.85rem;
    font-weight: 500;
    margin-bottom: 20px;
    color: var(--blue);
}

.hero-title {
    font-size: 3.5rem;
    font-weight: 900;
    line-height: 1.1;
    margin-bottom: 20px;
}

.title-line {
    display: block;
    color: var(--text-muted);
    font-size: 1.8rem;
    font-weight: 600;
}

.title-highlight {
    background: linear-gradient(135deg, var(--blue), var(--purple), var(--teal));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

.hero-subtitle {
    font-size: 1.1rem;
    color: var(--text-muted);
    line-height: 1.7;
    margin-bottom: 30px;
}

.hero-stats {
    display: flex;
    gap: 30px;
    margin-bottom: 35px;
}

.stat-item {
    display: flex;
    flex-direction: column;
}

.stat-number {
    font-size: 2rem;
    font-weight: 800;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
}

.stat-label {
    font-size: 0.8rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 1px;
}

.hero-cta {
    display: inline-flex;
    align-items: center;
    gap: 10px;
    padding: 14px 32px;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    border-radius: 99px;
    color: #fff;
    font-weight: 600;
    font-size: 1rem;
    transition: transform .3s, box-shadow .3s;
}

.hero-cta:hover {
    transform: translateY(-3px);
    box-shadow: 0 10px 40px var(--blue-glow);
}

.hero-visual {
    position: relative;
    width: 350px;
    height: 350px;
    flex-shrink: 0;
}

.floating-card {
    position: absolute;
    padding: 16px 24px;
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    display: flex;
    align-items: center;
    gap: 12px;
    box-shadow: 0 8px 30px #00000040;
    animation: floatCard 6s ease-in-out infinite;
}

.fc-icon {
    font-size: 1.5rem;
    font-weight: 800;
}

.fc-text {
    font-weight: 600;
    font-size: 0.9rem;
}

.card-1 {
    top: 20px;
    left: 20px;
    animation-delay: 0s;
    border-color: var(--blue-glow);
}

.card-1 .fc-icon {
    color: var(--blue);
}

.card-2 {
    top: 40px;
    right: 0;
    animation-delay: 1.5s;
    border-color: var(--purple-glow);
}

.card-2 .fc-icon {
    color: var(--purple);
}

.card-3 {
    bottom: 80px;
    left: 0;
    animation-delay: 3s;
    border-color: var(--teal-glow);
}

.card-3 .fc-icon {
    color: var(--teal);
}

.card-4 {
    bottom: 20px;
    right: 20px;
    animation-delay: 4.5s;
    border-color: var(--rose-glow);
}

.card-4 .fc-icon {
    color: var(--rose);
}

@keyframes floatCard {

    0%,
    100% {
        transform: translateY(0)
    }

    50% {
        transform: translateY(-15px)
    }
}

/* ===== MAIN CONTENT ===== */
.main-content {
    max-width: 900px;
    margin: 0 auto;
    padding: 0 24px 60px;
}

.chapter-section {
    margin-bottom: 80px;
}

/* Section Header */
.section-header {
    display: flex;
    align-items: center;
    gap: 20px;
    margin-bottom: 40px;
    padding: 30px;
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius);
}

.section-number {
    font-size: 3rem;
    font-weight: 900;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    line-height: 1;
}

.section-info {
    flex: 1;
}

.section-title {
    font-size: 1.8rem;
    font-weight: 800;
}

.section-desc {
    color: var(--text-muted);
    font-size: 0.95rem;
}

.section-icon {
    font-size: 2.5rem;
    font-weight: 800;
    color: var(--text-dim);
}

/* Content Cards */
.content-card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    margin-bottom: 24px;
    overflow: hidden;
    position: relative;
    opacity: 0;
    transform: translateY(30px);
    transition: opacity .6s, transform .6s;
}

.content-card.revealed {
    opacity: 1;
    transform: translateY(0);
}

.card-accent {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    width: 4px;
    background: var(--blue);
}

.accent-gradient {
    background: linear-gradient(180deg, var(--blue), var(--purple));
}

.accent-blue {
    background: var(--blue);
}

.accent-purple {
    background: var(--purple);
}

.accent-teal {
    background: var(--teal);
}

.accent-emerald {
    background: var(--emerald);
}

.accent-rose {
    background: var(--rose);
}

.accent-orange {
    background: var(--orange);
}

.accent-amber {
    background: var(--amber);
}

.card-body {
    padding: 32px;
}

.card-body h3 {
    font-size: 1.4rem;
    font-weight: 700;
    margin-bottom: 16px;
}

/* Key Concepts */
.key-concepts {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 16px;
    margin-top: 16px;
}

.concept-pill {
    display: flex;
    align-items: flex-start;
    gap: 14px;
    padding: 16px 20px;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.concept-pill.arithmetique {
    background: var(--blue-glow);
    border-color: #4d8ef740;
}

.concept-pill.geometrique {
    background: var(--purple-glow);
    border-color: #9b59f540;
}

.pill-icon {
    font-size: 1.5rem;
}

.concept-pill strong {
    display: block;
    margin-bottom: 2px;
}

.concept-pill span {
    font-size: 0.9rem;
    color: var(--text-muted);
}

/* Comparison Table */
.comparison-table-wrapper {
    overflow-x: auto;
}

.comparison-table {
    width: 100%;
    border-collapse: separate;
    border-spacing: 0;
    border-radius: var(--radius-sm);
    overflow: hidden;
    font-size: 0.9rem;
}

.comparison-table th,
.comparison-table td {
    padding: 14px 18px;
    text-align: left;
    border-bottom: 1px solid var(--border);
}

.comparison-table thead th {
    background: #0a0a1a;
    font-weight: 600;
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.col-arith {
    color: var(--blue);
}

.col-geo {
    color: var(--purple);
}

.feature-cell {
    display: flex;
    align-items: center;
    gap: 10px;
    font-weight: 500;
}

.feature-icon {
    font-size: 1.2rem;
}

.arith-cell {
    background: #4d8ef708;
}

.geo-cell {
    background: #9b59f508;
}

.comparison-table small {
    display: block;
    color: var(--text-muted);
    margin-top: 4px;
}

/* Explanation Blocks */
.explanation-block {
    margin-bottom: 28px;
    padding-bottom: 28px;
    border-bottom: 1px solid var(--border);
}

.explanation-block:last-child {
    border-bottom: none;
    margin-bottom: 0;
    padding-bottom: 0;
}

.exp-header {
    margin-bottom: 12px;
}

.exp-tag {
    display: inline-block;
    padding: 4px 14px;
    background: var(--blue);
    color: #fff;
    font-size: 0.75rem;
    font-weight: 700;
    border-radius: 99px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.tag-purple {
    background: var(--purple);
}

.tag-teal {
    background: var(--teal);
    color: #0a0a1a;
}

.tag-emerald {
    background: var(--emerald);
    color: #0a0a1a;
}

.tag-rose {
    background: var(--rose);
    color: #0a0a1a;
}

.interactive-tag {
    animation: pulse-tag 2s ease-in-out infinite;
}

@keyframes pulse-tag {

    0%,
    100% {
        box-shadow: 0 0 0 0 var(--blue-glow)
    }

    50% {
        box-shadow: 0 0 0 8px transparent
    }
}

/* Formula Box */
.formula-box {
    padding: 20px;
    background: #0d0d20;
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
    text-align: center;
    margin: 16px 0;
    position: relative;
    overflow: hidden;
}

.formula-box::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    height: 2px;
    background: linear-gradient(90deg, var(--blue), var(--purple));
}

.highlight-green::before {
    background: linear-gradient(90deg, var(--emerald), var(--teal));
}

.highlight-purple::before {
    background: linear-gradient(90deg, var(--purple), var(--rose));
}

.highlight-teal::before {
    background: linear-gradient(90deg, var(--teal), var(--emerald));
}

.highlight-emerald::before {
    background: linear-gradient(90deg, var(--emerald), var(--blue));
}

.highlight-rose::before {
    background: linear-gradient(90deg, var(--rose), var(--purple));
}

.formula {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1.3rem;
    font-weight: 600;
    color: #fff;
}

.big-formula .formula {
    font-size: 1.5rem;
}

.formula-note {
    display: block;
    margin-top: 8px;
    font-size: 0.85rem;
    color: var(--text-muted);
}

.tip-text {
    margin-top: 12px;
    padding: 12px 16px;
    background: #fbbf2410;
    border-left: 3px solid var(--amber);
    border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
    font-size: 0.9rem;
    color: var(--amber);
}

/* Example Boxes */
.example-box {
    margin-top: 16px;
    padding: 20px;
    background: #4d8ef710;
    border: 1px solid #4d8ef730;
    border-radius: var(--radius-sm);
}

.example-purple {
    background: #9b59f510;
    border-color: #9b59f530;
}

.example-orange {
    background: #fb923c10;
    border-color: #fb923c30;
}

.example-emerald {
    background: #34d39910;
    border-color: #34d39930;
}

.example-rose {
    background: #f472b610;
    border-color: #f472b630;
}

.example-title {
    font-weight: 700;
    font-size: 0.95rem;
    margin-bottom: 10px;
}

.example-solution .solution-content {
    display: none;
    margin-top: 12px;
    padding: 16px;
    background: #0a0a1a80;
    border-radius: var(--radius-sm);
}

.example-solution.show .solution-content {
    display: block;
    animation: fadeSlide .3s;
}

.example-solution.show .reveal-btn {
    display: none;
}

@keyframes fadeSlide {
    from {
        opacity: 0;
        transform: translateY(-10px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* Reveal Buttons */
.reveal-btn {
    padding: 10px 22px;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    color: #fff;
    border: none;
    border-radius: 99px;
    font-weight: 600;
    font-size: 0.85rem;
    cursor: pointer;
    transition: transform .2s, box-shadow .2s;
}

.reveal-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px var(--blue-glow);
}

.btn-purple {
    background: linear-gradient(135deg, var(--purple), var(--rose));
}

.btn-orange {
    background: linear-gradient(135deg, var(--orange), var(--amber));
}

.btn-emerald {
    background: linear-gradient(135deg, var(--emerald), var(--teal));
}

.btn-rose {
    background: linear-gradient(135deg, var(--rose), var(--purple));
}

/* Variation Grid */
.variation-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
    margin: 16px 0;
}

.variation-case {
    padding: 20px;
    text-align: center;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.var-arrow {
    font-size: 2rem;
    display: block;
    margin-bottom: 8px;
}

.var-desc {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-top: 4px;
}

.var-example {
    font-size: 0.8rem;
    color: var(--text-dim);
    margin-top: 6px;
}

.case-up {
    background: #34d39910;
    border-color: #34d39930;
}

.case-up .var-arrow {
    color: var(--emerald);
}

.case-const {
    background: #fbbf2410;
    border-color: #fbbf2430;
}

.case-const .var-arrow {
    color: var(--amber);
}

.case-down {
    background: #f472b610;
    border-color: #f472b630;
}

.case-down .var-arrow {
    color: var(--rose);
}

.case-up-purple {
    background: #9b59f510;
    border-color: #9b59f530;
}

.case-up-purple .var-arrow {
    color: var(--purple);
}

.case-const-purple {
    background: #fbbf2410;
    border-color: #fbbf2430;
}

.case-const-purple .var-arrow {
    color: var(--amber);
}

.case-down-purple {
    background: #f472b610;
    border-color: #f472b630;
}

.case-down-purple .var-arrow {
    color: var(--rose);
}

/* Warning Box */
.warning-box {
    display: flex;
    align-items: flex-start;
    gap: 12px;
    margin-top: 16px;
    padding: 16px;
    background: #fb923c10;
    border: 1px solid #fb923c30;
    border-radius: var(--radius-sm);
}

.warning-icon {
    font-size: 1.3rem;
    flex-shrink: 0;
}

/* Graph Controls & Canvas */
.graph-controls {
    display: flex;
    gap: 20px;
    margin-bottom: 16px;
    flex-wrap: wrap;
}

.control-group {
    display: flex;
    align-items: center;
    gap: 10px;
    flex: 1;
    min-width: 180px;
}

.control-group label {
    font-weight: 600;
    font-size: 0.9rem;
    min-width: 30px;
    color: var(--text-muted);
}

.control-group input[type="range"] {
    flex: 1;
    accent-color: var(--blue);
    height: 6px;
}

.control-value {
    font-family: 'JetBrains Mono', monospace;
    font-weight: 600;
    min-width: 40px;
    text-align: center;
    padding: 4px 8px;
    background: #0a0a1a;
    border-radius: 6px;
    font-size: 0.85rem;
}

canvas {
    width: 100%;
    border-radius: var(--radius-sm);
    background: #080818;
    border: 1px solid var(--border);
}

.graph-caption {
    text-align: center;
    margin-top: 10px;
    font-size: 0.88rem;
    color: var(--text-muted);
    font-style: italic;
    padding: 8px 16px;
    background: #ffffff06;
    border-radius: var(--radius-sm);
}

/* ===== LIVE FORMULA PANEL ===== */
.live-formula-panel {
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    gap: 12px;
    padding: 14px 20px;
    background: #4d8ef710;
    border: 1px solid #4d8ef730;
    border-radius: var(--radius-sm);
    margin-bottom: 16px;
}

.live-formula-purple {
    background: #9b59f510;
    border-color: #9b59f530;
}

.live-formula-label {
    font-size: 0.8rem;
    font-weight: 700;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    white-space: nowrap;
}

.live-formula-expr {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1.1rem;
    font-weight: 600;
    color: #fff;
    flex: 1;
    text-align: center;
}

.lf-u0,
.lf-v0 {
    display: inline-block;
    color: #4d8ef7;
    font-weight: 800;
    padding: 0 4px;
    border-bottom: 2px solid #4d8ef7;
    transition: all .3s;
}

.lf-r {
    display: inline-block;
    color: #34d399;
    font-weight: 800;
    padding: 0 4px;
    border-bottom: 2px solid #34d399;
    transition: all .3s;
}

.lf-q {
    display: inline-block;
    color: #f472b6;
    font-weight: 800;
    padding: 0 4px;
    border-bottom: 2px solid #f472b6;
    transition: all .3s;
}

.live-badge {
    display: inline-flex;
    align-items: center;
    gap: 4px;
    padding: 4px 12px;
    background: #34d39920;
    border: 1px solid #34d39940;
    border-radius: 99px;
    font-size: 0.78rem;
    font-weight: 700;
    color: var(--emerald);
    white-space: nowrap;
    transition: all .4s;
}

.live-badge-purple {
    background: #9b59f520;
    border-color: #9b59f540;
    color: var(--purple);
}

.live-badge.badge-down {
    background: #f472b620;
    border-color: #f472b640;
    color: var(--rose);
}

.live-badge.badge-const {
    background: #fbbf2420;
    border-color: #fbbf2440;
    color: var(--amber);
}

/* ===== LIVE VALUE TABLE ===== */
.live-values-table {
    margin-top: 14px;
    padding: 14px 16px;
    background: #0a0a2080;
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
}

.live-values-purple {
    border-color: #9b59f520;
}

.lvt-title {
    font-size: 0.78rem;
    font-weight: 700;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
    margin-bottom: 10px;
}

.lvt-row {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
}

.lvt-cell {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 3px;
    min-width: 52px;
    flex: 1;
}

.lvt-index {
    font-size: 0.7rem;
    color: var(--text-dim);
    font-weight: 600;
}

.lvt-value {
    font-family: 'JetBrains Mono', monospace;
    font-size: 0.82rem;
    font-weight: 700;
    padding: 5px 8px;
    border-radius: 8px;
    background: #4d8ef720;
    color: #4d8ef7;
    border: 1px solid #4d8ef730;
    text-align: center;
    transition: transform .2s, background .3s;
    width: 100%;
}

.lvt-value:hover {
    transform: scale(1.05);
}

.lvt-value.purple-val {
    background: #9b59f720;
    color: #9b59f5;
    border-color: #9b59f730;
}


.two-columns {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 20px;
    margin: 16px 0;
}

.column-card {
    padding: 20px;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.col-positive {
    background: #34d39908;
    border-color: #34d39930;
}

.col-negative {
    background: #f472b608;
    border-color: #f472b630;
}

.column-card h4 {
    font-size: 1rem;
    margin-bottom: 12px;
    text-align: center;
}

.mini-graph {
    margin-bottom: 12px;
}

.mini-graph canvas {
    width: 100%;
    height: 160px;
}

.property-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 6px;
    font-size: 0.9rem;
}

/* Info Callout */
.info-callout {
    display: flex;
    align-items: flex-start;
    gap: 14px;
    padding: 20px;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.callout-teal {
    background: #2dd4bf10;
    border-color: #2dd4bf30;
}

.callout-rose {
    background: #f472b610;
    border-color: #f472b630;
}

.callout-icon {
    font-size: 1.5rem;
    flex-shrink: 0;
}

.info-callout ul {
    margin-top: 8px;
    padding-left: 20px;
}

.info-callout li {
    margin-bottom: 4px;
}

/* Derivation Visual */
.derivation-visual {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 10px;
    margin: 20px 0;
    padding: 24px;
    background: #0a0a1a;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.deriv-from,
.deriv-to {
    display: flex;
    align-items: center;
    gap: 6px;
    flex-wrap: wrap;
    justify-content: center;
    font-family: 'JetBrains Mono', monospace;
    font-size: 1.1rem;
}

.deriv-label {
    font-weight: 700;
    color: var(--text-muted);
    margin-right: 8px;
}

.deriv-term {
    padding: 6px 12px;
    border-radius: 8px;
}

.t-a {
    background: #4d8ef720;
    color: var(--blue);
}

.t-b {
    background: #9b59f520;
    color: var(--purple);
}

.t-c {
    background: #2dd4bf20;
    color: var(--teal);
}

.t-d {
    background: #fb923c20;
    color: var(--orange);
}

.t-d-gone {
    background: #f472b620;
    color: var(--rose);
    font-size: 0.8rem;
    text-decoration: line-through;
    opacity: 0.6;
}

.deriv-op {
    color: var(--text-dim);
    font-weight: 600;
}

.deriv-arrow {
    display: flex;
    flex-direction: column;
    align-items: center;
    color: var(--text-dim);
    font-size: 0.8rem;
    font-weight: 600;
}

/* Rule Cards */
.rule-cards {
    display: flex;
    gap: 16px;
    margin-top: 16px;
    justify-content: center;
}

.rule-card {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 12px 20px;
    background: #0a0a1a;
    border: 1px solid var(--border);
    border-radius: var(--radius-sm);
}

.rule-card code {
    background: none;
    padding: 0;
}

.rule-arrow {
    color: var(--teal);
    font-weight: 700;
    font-size: 1.2rem;
}

/* Delta Cases */
.delta-cases {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
    margin-top: 16px;
}

.delta-case {
    padding: 18px;
    text-align: center;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}

.delta-case h4 {
    font-size: 1rem;
    margin-bottom: 8px;
}

.delta-case p {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-bottom: 4px;
}

.delta-shape {
    font-size: 0.8rem !important;
    font-style: italic;
    color: var(--text-dim) !important;
}

.delta-pos {
    background: #34d39910;
    border-color: #34d39930;
}

.delta-pos h4 {
    color: var(--emerald);
}

.delta-zero {
    background: #fbbf2410;
    border-color: #fbbf2430;
}

.delta-zero h4 {
    color: var(--amber);
}

.delta-neg {
    background: #f472b610;
    border-color: #f472b630;
}

.delta-neg h4 {
    color: var(--rose);
}

/* Sign Table */
.sign-table-visual {
    margin: 16px 0;
    border-radius: var(--radius-sm);
    overflow: hidden;
    border: 1px solid var(--border);
}

.sign-row {
    display: flex;
}

.sign-header-row {
    background: #0a0a1a;
    font-weight: 600;
}

.sign-cell {
    flex: 1;
    padding: 10px;
    text-align: center;
    font-size: 0.85rem;
    border-right: 1px solid var(--border);
}

.sign-cell:last-child {
    border-right: none;
}

.sign-label {
    font-weight: 600;
    background: #0a0a1a;
}

.sign-root {
    color: var(--amber);
    font-weight: 700;
}

.sign-pos {
    color: var(--emerald);
    font-weight: 700;
    font-size: 1.1rem;
}

.sign-neg {
    color: var(--rose);
    font-weight: 700;
    font-size: 1.1rem;
}

.sign-zero {
    color: var(--amber);
    font-weight: 700;
}

/* Method Steps */
.method-steps {
    display: flex;
    flex-direction: column;
    gap: 16px;
    margin-bottom: 24px;
}

.method-step {
    display: flex;
    gap: 16px;
    align-items: flex-start;
    padding: 20px;
    background: #0a0a1a40;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
    transition: border-color .3s;
}

.method-step:hover {
    border-color: #ffffff20;
}

.step-number {
    width: 40px;
    height: 40px;
    min-width: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    border-radius: 50%;
    font-weight: 800;
    color: #fff;
}

.step-content h4 {
    font-size: 1rem;
    margin-bottom: 6px;
}

.step-content p {
    font-size: 0.9rem;
    color: var(--text-muted);
}

.text-blue {
    color: var(--blue);
}

.text-purple {
    color: var(--purple);
}

/* ===== QUIZ ===== */
.quiz-container {
    margin-bottom: 40px;
}

.quiz-progress-bar {
    height: 6px;
    background: #1e1e3a;
    border-radius: 99px;
    overflow: hidden;
    margin-bottom: 12px;
}

.quiz-progress-fill {
    height: 100%;
    width: 10%;
    background: linear-gradient(90deg, var(--blue), var(--teal));
    border-radius: 99px;
    transition: width .5s;
}

.quiz-score-display {
    text-align: center;
    margin-bottom: 20px;
    font-size: 0.9rem;
    color: var(--text-muted);
    font-weight: 600;
}

.quiz-card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 40px;
}

.quiz-question {
    font-size: 1.2rem;
    font-weight: 700;
    margin-bottom: 24px;
    line-height: 1.5;
}

.quiz-options {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.quiz-option {
    padding: 16px 20px;
    background: #0a0a1a;
    border: 2px solid var(--border);
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: all .3s;
    font-size: 0.95rem;
    font-weight: 500;
}

.quiz-option:hover {
    border-color: var(--blue);
    background: var(--blue-glow);
}

.quiz-option.correct {
    border-color: var(--emerald);
    background: #34d39920;
    color: var(--emerald);
}

.quiz-option.wrong {
    border-color: var(--rose);
    background: #f472b620;
    color: var(--rose);
}

.quiz-option.disabled {
    pointer-events: none;
    opacity: 0.5;
}

.quiz-option.correct.disabled {
    opacity: 1;
}

.quiz-feedback {
    margin-top: 20px;
    padding: 16px;
    border-radius: var(--radius-sm);
    font-size: 0.9rem;
    display: none;
}

.quiz-feedback.show {
    display: block;
    animation: fadeSlide .3s;
}

.quiz-feedback.correct {
    background: #34d39910;
    border: 1px solid #34d39930;
    color: var(--emerald);
}

.quiz-feedback.wrong {
    background: #f472b610;
    border: 1px solid #f472b630;
    color: var(--rose);
}

.quiz-next-btn {
    margin-top: 20px;
    padding: 12px 28px;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    color: #fff;
    border: none;
    border-radius: 99px;
    font-weight: 600;
    cursor: pointer;
    transition: transform .2s;
}

.quiz-next-btn:hover {
    transform: translateY(-2px);
}

/* Quiz Results */
.quiz-results {
    text-align: center;
    padding: 60px 40px;
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius);
}

.results-circle {
    position: relative;
    width: 150px;
    height: 150px;
    margin: 0 auto 24px;
}

.results-circle svg {
    width: 100%;
    height: 100%;
    transform: rotate(-90deg);
}

.results-bg {
    fill: none;
    stroke: #1e1e3a;
    stroke-width: 8;
}

.results-fill {
    fill: none;
    stroke: url(#resultGradient);
    stroke-width: 8;
    stroke-linecap: round;
    stroke-dasharray: 339.292;
    stroke-dashoffset: 339.292;
    transition: stroke-dashoffset 1.5s ease-out;
}

.results-score {
    position: absolute;
    inset: 0;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2rem;
    font-weight: 800;
}

.results-title {
    font-size: 1.5rem;
    font-weight: 800;
    margin-bottom: 10px;
}

.results-message {
    color: var(--text-muted);
    margin-bottom: 30px;
}

.quiz-restart-btn {
    padding: 14px 32px;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    color: #fff;
    border: none;
    border-radius: 99px;
    font-weight: 600;
    font-size: 1rem;
    cursor: pointer;
    transition: transform .2s;
}

.quiz-restart-btn:hover {
    transform: translateY(-3px);
}

/* ===== FLASHCARDS ===== */
.flashcard-instructions {
    color: var(--text-muted);
    margin-bottom: 20px;
    font-size: 0.9rem;
}

.flashcards-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
    gap: 16px;
}

.flashcard {
    height: 180px;
    perspective: 800px;
    cursor: pointer;
}

.flashcard-inner {
    position: relative;
    width: 100%;
    height: 100%;
    transition: transform .6s;
    transform-style: preserve-3d;
}

.flashcard.flipped .flashcard-inner {
    transform: rotateY(180deg);
}

.flashcard-front,
.flashcard-back {
    position: absolute;
    inset: 0;
    backface-visibility: hidden;
    border-radius: var(--radius-sm);
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 24px;
    text-align: center;
}

.flashcard-front {
    background: linear-gradient(135deg, #1a1a3a, #12122a);
    border: 1px solid var(--border);
}

.flashcard-back {
    background: linear-gradient(135deg, var(--blue-glow), var(--purple-glow));
    border: 1px solid #4d8ef740;
    transform: rotateY(180deg);
}

.fc-q {
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--text-muted);
}

.fc-a {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1rem;
    font-weight: 600;
    color: #fff;
}

/* ===== CHEATSHEET ===== */
.cheatsheet-section {
    max-width: 900px;
    margin: 0 auto 80px;
    padding: 0 24px;
    opacity: 0;
    transform: translateY(30px);
    transition: opacity .6s, transform .6s;
}

.cheatsheet-section.revealed {
    opacity: 1;
    transform: translateY(0);
}

.cheatsheet-title {
    text-align: center;
    font-size: 1.8rem;
    font-weight: 800;
    margin-bottom: 30px;
}

.cheatsheet-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 16px;
}

.cheat-card {
    padding: 24px;
    border-radius: var(--radius-sm);
    text-align: center;
}

.cheat-card h4 {
    font-size: 0.9rem;
    margin-bottom: 12px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.cheat-formula {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1rem;
    padding: 6px;
    color: #fff;
    font-weight: 600;
}

.cheat-blue {
    background: #4d8ef715;
    border: 1px solid #4d8ef730;
}

.cheat-blue h4 {
    color: var(--blue);
}

.cheat-purple {
    background: #9b59f515;
    border: 1px solid #9b59f530;
}

.cheat-purple h4 {
    color: var(--purple);
}

.cheat-teal {
    background: #2dd4bf15;
    border: 1px solid #2dd4bf30;
}

.cheat-teal h4 {
    color: var(--teal);
}

.cheat-rose {
    background: #f472b615;
    border: 1px solid #f472b630;
}

.cheat-rose h4 {
    color: var(--rose);
}

/* ===== MOBILE BOTTOM NAV ===== */
.mobile-nav {
    display: none;
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
    z-index: 100;
    background: #0a0a1aee;
    backdrop-filter: blur(20px);
    border-top: 1px solid var(--border);
    padding: 8px 0;
    padding-bottom: env(safe-area-inset-bottom, 8px);
    justify-content: space-around;
}

.mobile-nav-item {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 2px;
    padding: 6px 16px;
    border-radius: 12px;
    color: var(--text-muted);
    text-decoration: none;
    font-size: 0.7rem;
    font-weight: 600;
    transition: color .3s, background .3s;
    min-width: 60px;
}

.mobile-nav-item.active {
    color: var(--blue);
    background: var(--blue-glow);
}

.mobile-nav-icon {
    font-size: 1.2rem;
    font-weight: 800;
}

.mobile-nav-label {
    font-size: 0.65rem;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

/* ===== FOOTER ===== */
.footer {
    text-align: center;
    padding: 40px;
    border-top: 1px solid var(--border);
    color: var(--text-muted);
    font-size: 0.9rem;
}

.scroll-top {
    position: fixed;
    bottom: 24px;
    right: 24px;
    width: 48px;
    height: 48px;
    border-radius: 50%;
    background: linear-gradient(135deg, var(--blue), var(--purple));
    color: #fff;
    border: none;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    opacity: 0;
    transform: translateY(20px);
    transition: opacity .3s, transform .3s;
    z-index: 50;
    box-shadow: 0 4px 20px var(--blue-glow);
}

.scroll-top.visible {
    opacity: 1;
    transform: translateY(0);
}

.scroll-top:hover {
    transform: translateY(-3px);
}

/* =====================================================
   RESPONSIVE — BASE (max 768px — tablettes & phones)
   ===================================================== */
@media (max-width: 768px) {

    /* --- Navbar --- */
    .navbar {
        padding: 10px 16px;
    }

    .nav-links {
        display: none;
    }

    .nav-progress {
        gap: 6px;
    }

    .progress-bar {
        width: 60px;
    }

    .nav-title {
        font-size: 0.9rem;
    }

    /* --- Hero --- */
    .hero {
        flex-direction: column;
        padding: 80px 16px 24px;
        text-align: center;
        min-height: auto;
        gap: 20px;
    }

    .hero-content {
        max-width: 100%;
    }

    .hero-title {
        font-size: 2rem;
    }

    .title-line {
        font-size: 1.15rem;
    }

    .hero-subtitle {
        font-size: 0.9rem;
        margin-bottom: 16px;
    }

    .hero-stats {
        justify-content: center;
        gap: 18px;
    }

    .stat-number {
        font-size: 1.5rem;
    }

    .hero-visual {
        display: none;
    }

    /* masqué sur mobile → économise de l'espace */
    .hero-cta {
        padding: 12px 22px;
        font-size: 0.9rem;
    }

    /* --- Main Layout --- */
    .main-content {
        padding: 0 10px 40px;
    }

    .chapter-section {
        margin-bottom: 40px;
    }

    /* --- Section Header --- */
    .section-header {
        flex-wrap: wrap;
        gap: 10px;
        padding: 16px;
    }

    .section-number {
        font-size: 1.8rem;
    }

    .section-title {
        font-size: 1.2rem;
    }

    .section-desc {
        font-size: 0.82rem;
    }

    .section-icon {
        font-size: 1.6rem;
    }

    /* --- Cards --- */
    .card-body {
        padding: 16px;
    }

    .card-body h3 {
        font-size: 1.05rem;
    }

    .explanation-block {
        margin-bottom: 18px;
        padding-bottom: 18px;
    }

    /* --- Grids → 1 colonne --- */
    .key-concepts,
    .two-columns,
    .cheatsheet-grid,
    .variation-grid,
    .delta-cases {
        grid-template-columns: 1fr;
    }

    /* --- Comparison Table : scrollable horizontalement --- */
    .comparison-table-wrapper {
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
        margin: 0 -16px;
        padding: 0 16px;
    }

    .comparison-table {
        min-width: 460px;
    }

    .comparison-table th,
    .comparison-table td {
        padding: 10px 10px;
        font-size: 0.78rem;
    }

    .feature-cell {
        flex-wrap: wrap;
        gap: 4px;
    }

    /* --- Formulas --- */
    .formula {
        font-size: 0.95rem;
    }

    .big-formula .formula {
        font-size: 1rem;
    }

    .formula-box {
        padding: 12px 14px;
    }

    .formula-note {
        font-size: 0.75rem;
    }

    /* --- Derivation --- */
    .derivation-visual {
        padding: 14px 8px;
        gap: 8px;
    }

    .deriv-from,
    .deriv-to {
        font-size: 0.78rem;
        gap: 4px;
        flex-wrap: wrap;
        justify-content: center;
    }

    .deriv-term {
        padding: 4px 7px;
        font-size: 0.75rem;
    }

    .deriv-label {
        font-size: 0.75rem;
    }

    /* --- Rule cards --- */
    .rule-cards {
        flex-direction: column;
        align-items: stretch;
    }

    .rule-card {
        justify-content: center;
    }

    /* --- Sign table --- */
    .sign-table-visual {
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
    }

    .sign-cell {
        padding: 6px 4px;
        font-size: 0.68rem;
    }

    /* --- Graph controls : sliders touch-friendly --- */
    .graph-controls {
        flex-direction: column;
        gap: 12px;
    }

    .control-group {
        min-width: 100%;
        background: #0a0a2060;
        border: 1px solid var(--border);
        border-radius: var(--radius-sm);
        padding: 10px 14px;
        display: flex;
        align-items: center;
        gap: 12px;
    }

    .control-group label {
        min-width: 40px;
        font-size: 1rem;
        font-weight: 700;
        color: var(--blue);
    }

    .control-group input[type="range"] {
        flex: 1;
        height: 36px;
        cursor: pointer;
        accent-color: var(--blue);
    }

    .control-value {
        min-width: 34px;
        text-align: center;
        font-size: 1rem;
        font-weight: 700;
        color: #fff;
    }

    /* --- Canvas : hauteur réduite sur mobile --- */
    canvas {
        width: 100% !important;
        height: 220px !important;
        border-radius: 8px;
        background: #0a0a2060;
    }

    /* --- Live Formula Panel --- */
    .live-formula-panel {
        flex-direction: column;
        align-items: flex-start;
        gap: 8px;
        padding: 12px 14px;
    }

    .live-formula-expr {
        font-size: 0.95rem;
        text-align: left;
        width: 100%;
    }

    .live-badge {
        font-size: 0.72rem;
        padding: 4px 10px;
    }

    /* --- Live Value Table --- */
    .live-values-table {
        overflow-x: auto;
        -webkit-overflow-scrolling: touch;
        padding: 10px 12px;
    }

    .lvt-row {
        flex-wrap: nowrap;
        width: max-content;
        gap: 6px;
    }

    .lvt-cell {
        min-width: 46px;
    }

    .lvt-value {
        font-size: 0.75rem;
        padding: 5px 6px;
    }

    .lvt-index {
        font-size: 0.65rem;
    }

    .lvt-title {
        font-size: 0.7rem;
    }

    /* --- Graph caption --- */
    .graph-caption {
        font-size: 0.8rem;
        padding: 8px 10px;
    }

    /* --- Quiz --- */
    .quiz-card {
        padding: 18px;
    }

    .quiz-question {
        font-size: 0.95rem;
        margin-bottom: 14px;
        line-height: 1.5;
    }

    .quiz-option {
        padding: 14px 16px;
        font-size: 0.88rem;
    }

    .quiz-results {
        padding: 32px 16px;
    }

    /* --- Flashcards : en pleine largeur --- */
    .flashcards-grid {
        grid-template-columns: 1fr;
    }

    .flashcard {
        height: 150px;
    }

    .fc-q {
        font-size: 0.82rem;
    }

    .fc-a {
        font-size: 0.88rem;
    }

    /* --- Cheatsheet --- */
    .cheatsheet-title {
        font-size: 1.2rem;
    }

    .cheat-card {
        padding: 14px;
    }

    .cheat-formula {
        font-size: 0.82rem;
    }

    /* --- Footer --- */
    .footer {
        padding: 20px 16px 90px;
        font-size: 0.8rem;
    }

    /* --- Scroll top --- */
    .scroll-top {
        width: 42px;
        height: 42px;
        bottom: 76px;
        right: 14px;
    }

    /* --- Callouts & Examples --- */
    .info-callout {
        flex-direction: column;
        gap: 8px;
        padding: 12px;
    }

    .example-box {
        padding: 12px;
    }

    .tip-text {
        font-size: 0.8rem;
        padding: 8px 12px;
    }

    .warning-box {
        padding: 10px;
        flex-direction: column;
        gap: 8px;
    }

    /* --- Method steps --- */
    .method-step {
        padding: 12px;
        gap: 10px;
    }

    .step-number {
        width: 32px;
        height: 32px;
        min-width: 32px;
        font-size: 0.82rem;
    }

    .step-content h4 {
        font-size: 0.9rem;
    }

    .step-content p {
        font-size: 0.82rem;
    }

    /* --- Mobile bottom nav : ON --- */
    .mobile-nav {
        display: flex;
    }

    body {
        padding-bottom: 70px;
    }
}

/* =====================================================
   RESPONSIVE — SMALL PHONE (max 480px)
   ===================================================== */
@media (max-width: 480px) {

    /* --- Navbar --- */
    .navbar {
        padding: 8px 12px;
    }

    .nav-icon {
        font-size: 1.15rem;
    }

    .nav-title {
        font-size: 0.8rem;
    }

    .progress-bar {
        width: 44px;
    }

    .progress-text {
        font-size: 0.68rem;
        min-width: 22px;
    }

    /* --- Hero --- */
    .hero {
        padding: 72px 12px 20px;
        gap: 16px;
    }

    .hero-badge {
        font-size: 0.72rem;
        padding: 4px 12px;
    }

    .hero-title {
        font-size: 1.65rem;
        line-height: 1.2;
    }

    .title-line {
        font-size: 0.95rem;
    }

    .hero-subtitle {
        font-size: 0.82rem;
    }

    .hero-stats {
        gap: 14px;
    }

    .stat-number {
        font-size: 1.25rem;
    }

    .stat-label {
        font-size: 0.62rem;
    }

    .hero-cta {
        padding: 11px 18px;
        font-size: 0.82rem;
        gap: 6px;
    }

    /* --- Main --- */
    .main-content {
        padding: 0 6px 30px;
    }

    .section-header {
        padding: 12px;
        gap: 8px;
    }

    .section-number {
        font-size: 1.5rem;
    }

    .section-title {
        font-size: 1rem;
    }

    .section-icon {
        font-size: 1.3rem;
    }

    /* --- Cards --- */
    .card-body {
        padding: 12px;
    }

    .card-body h3 {
        font-size: 0.95rem;
        margin-bottom: 10px;
    }

    .explanation-block {
        margin-bottom: 14px;
        padding-bottom: 14px;
    }

    /* --- Formulas --- */
    .formula {
        font-size: 0.82rem;
        word-break: break-word;
    }

    .big-formula .formula {
        font-size: 0.9rem;
    }

    .formula-box {
        padding: 10px 8px;
    }

    code {
        font-size: 0.78em;
        padding: 1px 5px;
    }

    /* --- Concept pills --- */
    .concept-pill {
        padding: 10px 12px;
        gap: 8px;
    }

    .pill-icon {
        font-size: 1.1rem;
    }

    .concept-pill span {
        font-size: 0.78rem;
    }

    /* --- Comparison Table --- */
    .comparison-table th,
    .comparison-table td {
        padding: 8px 6px;
        font-size: 0.7rem;
    }

    .feature-icon {
        font-size: 0.85rem;
    }

    /* --- Variation cases --- */
    .variation-case {
        padding: 12px 8px;
    }

    .var-arrow {
        font-size: 1.4rem;
    }

    .var-desc {
        font-size: 0.72rem;
    }

    .var-example {
        font-size: 0.67rem;
    }

    /* --- Derivation --- */
    .derivation-visual {
        padding: 10px 6px;
    }

    .deriv-from,
    .deriv-to {
        font-size: 0.7rem;
    }

    .deriv-term {
        padding: 2px 5px;
        font-size: 0.68rem;
    }

    .deriv-label {
        font-size: 0.68rem;
    }

    .t-d-gone {
        font-size: 0.62rem;
    }

    .deriv-arrow svg {
        width: 26px;
        height: 26px;
    }

    /* --- Rule cards --- */
    .rule-card {
        padding: 8px 12px;
        font-size: 0.78rem;
    }

    /* --- Delta cases --- */
    .delta-case {
        padding: 10px 8px;
    }

    .delta-case h4 {
        font-size: 0.82rem;
    }

    .delta-case p {
        font-size: 0.72rem;
    }

    /* --- Sign table --- */
    .sign-cell {
        padding: 4px 2px;
        font-size: 0.58rem;
    }

    /* --- Graph controls --- */
    .control-group {
        padding: 8px 10px;
        gap: 8px;
    }

    .control-group label {
        font-size: 0.9rem;
        min-width: 32px;
    }

    .control-group input[type="range"] {
        height: 32px;
    }

    .control-value {
        font-size: 0.9rem;
        min-width: 28px;
    }

    /* --- Canvas --- */
    canvas {
        height: 200px !important;
    }

    /* --- Live Formula Panel --- */
    .live-formula-panel {
        padding: 10px 12px;
        gap: 6px;
    }

    .live-formula-label {
        font-size: 0.7rem;
    }

    .live-formula-expr {
        font-size: 0.85rem;
    }

    .lf-u0,
    .lf-v0,
    .lf-r,
    .lf-q {
        font-size: 0.9rem;
    }

    .live-badge {
        font-size: 0.68rem;
        padding: 3px 8px;
    }

    /* --- Live Value Table --- */
    .lvt-cell {
        min-width: 40px;
    }

    .lvt-value {
        font-size: 0.7rem;
        padding: 4px 4px;
    }

    .lvt-index {
        font-size: 0.6rem;
    }

    /* --- Quiz --- */
    .quiz-card {
        padding: 14px;
    }

    .quiz-question {
        font-size: 0.88rem;
        margin-bottom: 12px;
    }

    .quiz-option {
        padding: 12px 12px;
        font-size: 0.8rem;
    }

    .quiz-feedback {
        font-size: 0.78rem;
        padding: 10px;
    }

    .quiz-next-btn {
        padding: 10px 18px;
        font-size: 0.82rem;
    }

    .quiz-results {
        padding: 26px 12px;
    }

    .results-circle {
        width: 110px;
        height: 110px;
    }

    .results-score {
        font-size: 1.4rem;
    }

    .results-title {
        font-size: 1.1rem;
    }

    .results-message {
        font-size: 0.82rem;
    }

    .quiz-restart-btn {
        padding: 10px 20px;
        font-size: 0.82rem;
    }

    /* --- Flashcards --- */
    .flashcard {
        height: 140px;
    }

    .flashcard-front,
    .flashcard-back {
        padding: 14px;
    }

    .fc-q {
        font-size: 0.78rem;
    }

    .fc-a {
        font-size: 0.82rem;
    }

    /* --- Cheatsheet --- */
    .cheatsheet-title {
        font-size: 1rem;
        margin-bottom: 16px;
    }

    .cheat-card {
        padding: 12px;
    }

    .cheat-card h4 {
        font-size: 0.72rem;
    }

    .cheat-formula {
        font-size: 0.75rem;
    }

    /* --- Buttons --- */
    .reveal-btn {
        padding: 8px 16px;
        font-size: 0.78rem;
    }

    .example-title {
        font-size: 0.82rem;
    }

    .example-box {
        padding: 10px;
    }

    .example-box p,
    .solution-content p {
        font-size: 0.8rem;
    }

    /* --- Callouts --- */
    .info-callout {
        padding: 10px;
    }

    .callout-icon {
        font-size: 1.1rem;
    }

    .info-callout p,
    .info-callout li {
        font-size: 0.8rem;
    }

    .tip-text {
        font-size: 0.75rem;
        padding: 7px 10px;
    }

    .warning-box {
        font-size: 0.78rem;
        padding: 9px;
    }

    /* --- Footer & scroll top --- */
    .footer {
        padding: 16px 12px 88px;
        font-size: 0.72rem;
    }

    .scroll-top {
        width: 38px;
        height: 38px;
        bottom: 74px;
        right: 10px;
    }

    .scroll-top svg {
        width: 18px;
        height: 18px;
    }

    /* --- Method steps --- */
    .method-step {
        padding: 10px 8px;
        gap: 8px;
    }

    .step-number {
        width: 28px;
        height: 28px;
        min-width: 28px;
        font-size: 0.72rem;
    }

    /* --- Mobile nav items --- */
    .mobile-nav-icon {
        font-size: 1.1rem;
    }

    .mobile-nav-label {
        font-size: 0.6rem;
    }

    .mobile-nav-item {
        padding: 6px 10px;
        min-width: 50px;
    }

    /* --- Graph caption --- */
    .graph-caption {
        font-size: 0.75rem;
    }
}

/* =====================================================
   TOUCH DEVICE ENHANCEMENTS
   Bigger tap targets, no sticky hover states
   ===================================================== */
@media (hover: none) and (pointer: coarse) {

    /* Tap targets ≥ 44px (Apple HIG / Google Material) */
    .quiz-option {
        min-height: 48px;
    }

    .reveal-btn {
        min-height: 44px;
        padding: 12px 22px;
    }

    .hero-cta {
        min-height: 48px;
    }

    .quiz-next-btn {
        min-height: 44px;
    }

    .quiz-restart-btn {
        min-height: 48px;
    }

    .scroll-top {
        min-width: 48px;
        min-height: 48px;
    }

    .mobile-nav-item {
        min-height: 52px;
        padding: 8px 14px;
    }

    .control-group input[type="range"] {
        min-height: 40px;
    }

    /* Remove sticky :hover states */
    .quiz-option:hover {
        border-color: var(--border);
        background: #0a0a1a;
    }

    .method-step:hover {
        border-color: var(--border);
    }

    .reveal-btn:hover {
        transform: none;
        box-shadow: none;
    }

    .content-card:hover {
        transform: none;
    }

    .flashcard:hover {
        cursor: default;
    }

    /* Tactile :active feedback */
    .quiz-option:active {
        border-color: var(--blue);
        background: var(--blue-glow);
        transform: scale(0.98);
    }

    .reveal-btn:active {
        transform: scale(0.96);
        opacity: 0.85;
    }

    .hero-cta:active {
        transform: scale(0.97);
    }

    .flashcard:active .flashcard-inner {
        transform: scale(0.98);
    }

    .mobile-nav-item:active {
        opacity: 0.7;
    }
}