:root {
    --bg: #f6f3ea;
    --paper: #fffaf0;
    --ink: #1e2430;
    --muted: #657084;
    --line: rgba(30, 36, 48, .12);
    --primary: #315eea;
    --primary-dark: #2447b8;
    --success: #13795b;
    --danger: #b42318;
    --warning: #9a6700;
    --shadow: 0 18px 50px rgba(31, 45, 75, .10);
    --radius: 24px;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
    margin: 0;
    font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
    color: var(--ink);
    background:
        radial-gradient(circle at top left, rgba(49, 94, 234, .12), transparent 34rem),
        radial-gradient(circle at bottom right, rgba(19, 121, 91, .10), transparent 30rem),
        var(--bg);
    min-height: 100vh;
}
a { color: inherit; text-decoration: none; }
p { line-height: 1.65; }

.topbar {
    position: sticky;
    top: 0;
    z-index: 5;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    padding: 1rem clamp(1rem, 4vw, 3rem);
    background: rgba(246, 243, 234, .86);
    backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--line);
}
.brand {
    display: flex;
    align-items: center;
    gap: .75rem;
    font-weight: 850;
    letter-spacing: -.03em;
    font-size: 1.15rem;
}
.brand-mark {
    display: grid;
    place-items: center;
    width: 2.25rem;
    height: 2.25rem;
    border-radius: .8rem;
    background: var(--ink);
    color: white;
    font-weight: 900;
}
.nav { display: flex; flex-wrap: wrap; gap: .35rem; justify-content: flex-end; }
.nav a {
    padding: .55rem .8rem;
    border-radius: 999px;
    color: var(--muted);
    font-weight: 650;
}
.nav a:hover { background: rgba(49, 94, 234, .10); color: var(--primary-dark); }

.container {
    width: min(1180px, calc(100% - 2rem));
    margin: 0 auto;
    padding: 2rem 0 4rem;
}
.hero {
    display: grid;
    grid-template-columns: 1.35fr .65fr;
    gap: 1.5rem;
    align-items: stretch;
    padding: 2rem 0;
}
.hero-content, .hero-card, .card, .auth-box {
    background: rgba(255, 250, 240, .82);
    border: 1px solid var(--line);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
}
.hero-content { padding: clamp(2rem, 5vw, 4rem); }
.hero-card { padding: 2rem; }
.eyebrow {
    margin: 0 0 .7rem;
    color: var(--primary-dark);
    text-transform: uppercase;
    letter-spacing: .14em;
    font-weight: 850;
    font-size: .78rem;
}
h1 {
    margin: 0 0 1rem;
    font-size: clamp(2.2rem, 5vw, 4.6rem);
    line-height: .98;
    letter-spacing: -.07em;
}
h2 { margin: 0 0 .8rem; letter-spacing: -.03em; }
h3 { margin-bottom: .35rem; }
.lead { color: var(--muted); font-size: 1.15rem; max-width: 60rem; }
.muted { color: var(--muted); }
.small-text { font-size: .9rem; }

.hero-actions { display: flex; flex-wrap: wrap; gap: .75rem; margin-top: 1.5rem; }
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--line);
    border-radius: 999px;
    padding: .82rem 1.1rem;
    font-weight: 800;
    background: white;
    cursor: pointer;
    transition: transform .15s ease, box-shadow .15s ease, background .15s ease;
}
.btn:hover { transform: translateY(-1px); box-shadow: 0 10px 24px rgba(31, 45, 75, .12); }
.btn.primary { background: var(--primary); color: white; border-color: var(--primary); }
.btn.primary:hover { background: var(--primary-dark); }
.btn.small { padding: .62rem .9rem; font-size: .93rem; }
.btn.mini { padding: .42rem .66rem; font-size: .82rem; }

.clean-list { list-style: none; padding: 0; margin: 1rem 0 0; display: grid; gap: .8rem; }
.clean-list li { display: flex; gap: .7rem; align-items: center; }
.clean-list span { color: var(--success); font-weight: 900; }
.grid { display: grid; gap: 1rem; margin-top: 1rem; }
.grid.two { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.card { padding: 1.4rem; }
.subject { min-height: 210px; }
.maths-card { background: linear-gradient(145deg, rgba(255,250,240,.92), rgba(222,230,255,.78)); }
.svt-card { background: linear-gradient(145deg, rgba(255,250,240,.92), rgba(222,245,236,.78)); }
.pill {
    display: inline-flex;
    width: fit-content;
    padding: .35rem .65rem;
    border-radius: 999px;
    background: rgba(49, 94, 234, .10);
    color: var(--primary-dark);
    font-size: .8rem;
    font-weight: 850;
}
.danger-pill { background: rgba(180, 35, 24, .10); color: var(--danger); }
.card-topline, .section-title {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    margin-bottom: .8rem;
}

.page-head { padding: 1.5rem 0; }
.stats-grid { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 1rem; margin-bottom: 1rem; }
.stat-card {
    padding: 1.2rem;
    background: var(--paper);
    border: 1px solid var(--line);
    border-radius: 1.3rem;
    box-shadow: var(--shadow);
}
.stat-card span { display: block; color: var(--muted); font-weight: 700; }
.stat-card strong { display: block; font-size: 2rem; margin-top: .4rem; letter-spacing: -.05em; }
.danger-soft { background: rgba(255, 235, 233, .85); }
.action { display: block; }
.action:hover { transform: translateY(-2px); }
.big-icon { font-size: 2.4rem; display: inline-block; margin-bottom: .6rem; }
.table-wrap { overflow-x: auto; }
table { width: 100%; border-collapse: collapse; }
th, td { text-align: left; padding: .85rem; border-bottom: 1px solid var(--line); vertical-align: middle; }
th { color: var(--muted); font-size: .86rem; }
.progress { height: .65rem; background: rgba(31, 45, 75, .10); border-radius: 999px; overflow: hidden; min-width: 120px; }
.progress span { display: block; height: 100%; background: var(--primary); border-radius: 999px; }

.auth-box { max-width: 520px; margin: 2rem auto; padding: 2rem; }
.form { display: grid; gap: 1rem; margin-top: 1rem; }
label { display: grid; gap: .4rem; font-weight: 750; }
input[type="text"], input[type="email"], input[type="password"], textarea {
    width: 100%;
    border: 1px solid var(--line);
    border-radius: 1rem;
    padding: .9rem 1rem;
    background: white;
    font: inherit;
    color: var(--ink);
}
textarea { resize: vertical; }
.flash { padding: .9rem 1rem; border-radius: 1rem; margin: .8rem 0; font-weight: 700; }
.flash.danger { background: rgba(180, 35, 24, .10); color: var(--danger); }
.flash.warning { background: rgba(154, 103, 0, .12); color: var(--warning); }
.flash.success { background: rgba(19, 121, 91, .12); color: var(--success); }

.exercise-layout { display: grid; grid-template-columns: 300px 1fr; gap: 1rem; align-items: start; }
.exercise-nav { position: sticky; top: 5.5rem; }
.exercise-list { padding-left: 1.2rem; display: grid; gap: .45rem; }
.exercise-list a { color: var(--muted); font-weight: 700; }
.exercise-list .active a { color: var(--primary-dark); }
.exercise-card { padding: clamp(1.2rem, 3vw, 2rem); }
.statement {
    padding: 1rem;
    border: 1px solid var(--line);
    background: white;
    border-radius: 1.2rem;
    margin: 1rem 0;
}
.formula-box {
    background: #101828;
    color: white;
    padding: 1rem;
    border-radius: 1.2rem;
    margin: 1rem 0;
}
.formula-box p { margin: .35rem 0; }
.schema-box {
    color: var(--ink);
    background: white;
    border: 1px solid var(--line);
    border-radius: 1.2rem;
    padding: 1rem;
    margin: 1rem 0;
}
.schema { width: 100%; height: auto; display: block; }
.answer-form { display: grid; gap: 1rem; margin: 1.2rem 0; }
.choices { border: 0; padding: 0; margin: 0; display: grid; gap: .7rem; }
.choices legend { font-weight: 850; margin-bottom: .5rem; }
.choice {
    display: flex;
    align-items: center;
    gap: .75rem;
    padding: .9rem 1rem;
    border: 1px solid var(--line);
    background: white;
    border-radius: 1rem;
    cursor: pointer;
}
.choice:hover { border-color: rgba(49, 94, 234, .35); }
.result-card {
    margin-top: 1rem;
    padding: 1.2rem;
    border-radius: 1.2rem;
    border: 1px solid var(--line);
}
.result-card.ok { background: rgba(19, 121, 91, .10); }
.result-card.ko { background: rgba(180, 35, 24, .08); }
.correction {
    background: rgba(255,255,255,.7);
    border: 1px solid var(--line);
    border-radius: 1rem;
    padding: 1rem;
    margin: .8rem 0;
}
details { margin-top: .8rem; }
summary { cursor: pointer; font-weight: 850; }
.result-actions { display: flex; flex-wrap: wrap; gap: .7rem; margin-top: 1rem; }
.attempt-history { margin-top: 1.2rem; }
.chips { display: flex; flex-wrap: wrap; gap: .5rem; }
.chip, .status {
    display: inline-flex;
    padding: .35rem .6rem;
    border-radius: 999px;
    font-size: .82rem;
    font-weight: 850;
    background: rgba(31,45,75,.08);
}
.chip.success, .status.success { background: rgba(19,121,91,.12); color: var(--success); }
.chip.danger, .status.danger { background: rgba(180,35,24,.10); color: var(--danger); }
.status.todo { color: var(--muted); }
.error-item { border-left: 6px solid rgba(180,35,24,.45); }
.success-panel { background: rgba(19,121,91,.10); }
.footer {
    width: min(1180px, calc(100% - 2rem));
    margin: 0 auto;
    padding: 1.5rem 0 2rem;
    border-top: 1px solid var(--line);
    display: flex;
    justify-content: space-between;
    gap: 1rem;
    color: var(--muted);
    font-size: .9rem;
}

@media (max-width: 900px) {
    .hero, .grid.two, .grid.three, .stats-grid, .exercise-layout { grid-template-columns: 1fr; }
    .exercise-nav { position: static; }
    h1 { font-size: 2.5rem; }
    .topbar { align-items: flex-start; flex-direction: column; }
    .footer { flex-direction: column; }
}

/* V2.1 */
.grid.four { grid-template-columns: repeat(4, minmax(0, 1fr)); }
.stack-form { display: grid; gap: 1rem; }
.stack-form select,
.stack-form input,
.stack-form textarea { width: 100%; }
.narrow.wide-form { max-width: 980px; margin: 0 auto; }
.formula-list h2 { margin-top: 2rem; }
.formula-list h2:first-child { margin-top: 0; }
.formula-group { display: grid; gap: .9rem; }
.formula-item { border: 1px solid rgba(15, 23, 42, .12); border-radius: 18px; padding: 1rem; background: rgba(255,255,255,.62); }
.session-banner { border: 1px solid rgba(37, 99, 235, .22); background: linear-gradient(135deg, rgba(37, 99, 235, .10), rgba(245, 158, 11, .08)); }
.mini-session { margin: .75rem 0 1rem; padding: .75rem 1rem; border-radius: 16px; display: flex; justify-content: space-between; gap: 1rem; align-items: center; }
.inline-form { display: inline; }
.inline-form .btn { white-space: nowrap; }
code { background: rgba(15,23,42,.08); padding: .12rem .32rem; border-radius: .35rem; }
.status { display: inline-flex; align-items: center; padding: .25rem .55rem; border-radius: 999px; font-weight: 800; font-size: .85rem; }
.status.success { background: rgba(34,197,94,.12); color: #166534; }
.status.danger { background: rgba(239,68,68,.12); color: #991b1b; }

@media (max-width: 1100px) {
  .grid.four { grid-template-columns: repeat(2, minmax(0, 1fr)); }
}
@media (max-width: 720px) {
  .grid.four { grid-template-columns: 1fr; }
  .mini-session { flex-direction: column; align-items: flex-start; }
}