:root {
  --bg: #f6f7fb;
  --card: rgba(255,255,255,.92);
  --text: #171821;
  --muted: #6c7080;
  --line: #e7e9f1;
  --primary: #5b5bd6;
  --primary-2: #7b6df2;
  --success: #16885f;
  --danger: #c74646;
  --warning: #a46b00;
  --shadow: 0 18px 50px rgba(39,43,88,.10);
  --radius: 24px;
}

* { box-sizing: border-box; }
html { min-height: 100%; background: var(--bg); }
body {
  margin: 0;
  min-height: 100vh;
  font-family: Inter, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  color: var(--text);
  background:
    radial-gradient(circle at 8% 0%, rgba(123,109,242,.12), transparent 28%),
    radial-gradient(circle at 90% 4%, rgba(91,91,214,.10), transparent 24%),
    var(--bg);
}
button, input, textarea { font: inherit; }
button { cursor: pointer; }
a { color: inherit; }

.shell { min-height: 100vh; }
.topbar {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 14px 22px;
  border-bottom: 1px solid rgba(231,233,241,.9);
  background: rgba(246,247,251,.82);
  backdrop-filter: blur(18px);
}
.brand { display: flex; align-items: center; gap: 12px; font-weight: 800; }
.brand-mark {
  width: 40px; height: 40px; display: grid; place-items: center;
  border-radius: 14px; color: #fff; font-weight: 800;
  background: linear-gradient(135deg, var(--primary), var(--primary-2));
  box-shadow: 0 10px 28px rgba(91,91,214,.28);
}
.brand-copy small { display: block; font-size: 11px; color: var(--muted); font-weight: 600; margin-top: 2px; }
.userbar { display: flex; align-items: center; gap: 10px; }
.userpill {
  padding: 8px 12px; background: #fff; border: 1px solid var(--line);
  border-radius: 999px; font-size: 13px; color: var(--muted);
}

.app-body { max-width: 1180px; margin: 0 auto; padding: 28px 22px 64px; }
.page-head { display: flex; justify-content: space-between; align-items: end; gap: 20px; margin-bottom: 22px; }
.page-head h1 { margin: 0; font-size: clamp(28px, 5vw, 42px); letter-spacing: -.04em; }
.page-head p { margin: 8px 0 0; color: var(--muted); max-width: 720px; line-height: 1.6; }

.nav { display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 24px; }
.nav button {
  border: 1px solid var(--line); background: rgba(255,255,255,.8); color: var(--muted);
  border-radius: 14px; padding: 10px 14px; font-weight: 700;
}
.nav button.active { background: var(--text); color: #fff; border-color: var(--text); }

.grid { display: grid; grid-template-columns: repeat(3, minmax(0,1fr)); gap: 18px; }
.card {
  background: var(--card); border: 1px solid rgba(231,233,241,.95);
  border-radius: var(--radius); box-shadow: var(--shadow); padding: 22px;
}
.hw-card { min-height: 230px; display: flex; flex-direction: column; }
.hw-card .icon {
  width: 48px; height: 48px; display: grid; place-items: center; border-radius: 16px;
  background: #f0efff; color: var(--primary); font-size: 24px; margin-bottom: 18px;
}
.hw-card h3 { margin: 0; font-size: 21px; }
.hw-card p { color: var(--muted); line-height: 1.55; font-size: 14px; flex: 1; }
.stat-row { display: flex; justify-content: space-between; gap: 12px; color: var(--muted); font-size: 13px; margin: 14px 0 8px; }
.progress { height: 9px; background: #eceef5; border-radius: 999px; overflow: hidden; }
.progress > span { display: block; height: 100%; width: 0; background: linear-gradient(90deg, var(--primary), var(--primary-2)); border-radius: inherit; }

.btn {
  border: 0; border-radius: 14px; padding: 12px 16px; font-weight: 800;
  background: var(--text); color: #fff; display: inline-flex; align-items: center; justify-content: center; gap: 8px;
}
.btn:hover { transform: translateY(-1px); }
.btn.primary { background: linear-gradient(135deg, var(--primary), var(--primary-2)); }
.btn.ghost { background: #fff; color: var(--text); border: 1px solid var(--line); }
.btn.success { background: var(--success); }
.btn.danger { background: var(--danger); }
.btn.small { padding: 9px 12px; border-radius: 12px; font-size: 13px; }
.btn:disabled { opacity: .55; cursor: not-allowed; transform: none; }

.auth-wrap { min-height: 100vh; display: grid; grid-template-columns: 1.08fr .92fr; }
.auth-hero { padding: 64px; display: flex; flex-direction: column; justify-content: space-between; color: #fff;
  background: linear-gradient(145deg, #24244f 0%, #4c4dbf 58%, #786cf1 100%); position: relative; overflow: hidden; }
.auth-hero::after { content:""; position:absolute; width:420px; height:420px; border-radius:50%; right:-140px; top:-80px; background:rgba(255,255,255,.08); }
.auth-hero h1 { font-size: clamp(42px, 7vw, 76px); line-height: .98; letter-spacing: -.06em; margin: 0; max-width: 760px; }
.auth-hero p { max-width: 610px; color: rgba(255,255,255,.78); line-height: 1.7; font-size: 17px; }
.hero-points { display: grid; gap: 12px; margin-top: 28px; }
.hero-point { display:flex; gap:10px; align-items:center; color:rgba(255,255,255,.9); }
.auth-panel { display: grid; place-items: center; padding: 36px; }
.auth-card { width: min(460px,100%); background:#fff; border:1px solid var(--line); border-radius:30px; padding:30px; box-shadow: var(--shadow); }
.auth-card h2 { margin: 0 0 8px; font-size: 30px; letter-spacing: -.03em; }
.auth-card > p { color: var(--muted); margin: 0 0 24px; line-height: 1.5; }
.tabs { display:flex; background:#f2f3f8; padding:4px; border-radius:14px; margin-bottom:20px; }
.tabs button { flex:1; border:0; background:transparent; padding:10px 12px; border-radius:11px; font-weight:800; color:var(--muted); }
.tabs button.active { background:#fff; color:var(--text); box-shadow:0 4px 14px rgba(24,25,37,.07); }
.field { display:grid; gap:7px; margin-bottom:14px; }
.field label { font-size:13px; font-weight:800; }
.input, .textarea {
  width:100%; border:1px solid var(--line); background:#fff; border-radius:14px; padding:13px 14px; outline:none; color:var(--text);
}
.input:focus, .textarea:focus { border-color: #aaaaf2; box-shadow:0 0 0 4px rgba(91,91,214,.09); }
.textarea { min-height:110px; resize:vertical; }
.auth-note { margin-top:16px; padding:13px 14px; border-radius:14px; background:#f7f7ff; color:#5b5f77; font-size:12px; line-height:1.55; }
.demo-actions { display:grid; grid-template-columns:1fr 1fr; gap:8px; margin-top:12px; }

.solver-wrap { max-width: 880px; margin: 0 auto; }
.solver-top { display:flex; justify-content:space-between; gap:16px; align-items:center; margin-bottom:16px; }
.solver-card { padding: clamp(22px,4vw,36px); }
.q-meta { display:flex; align-items:center; justify-content:space-between; gap:12px; color:var(--muted); font-size:13px; }
.q-title { margin: 24px 0 10px; font-size: 19px; }
.formula { min-height:100px; display:grid; place-items:center; text-align:center; font-size: clamp(23px,4vw,36px); padding:20px; background:#fafaff; border:1px solid #ecebff; border-radius:20px; overflow-x:auto; }
.answer-row { display:flex; gap:10px; margin-top:20px; }
.answer-row .input { flex:1; font-size:18px; }
.feedback { margin-top:18px; border-radius:18px; padding:18px; line-height:1.65; }
.feedback.correct { background:#edf9f4; border:1px solid #ccecdf; }
.feedback.wrong { background:#fff3f3; border:1px solid #ffd9d9; }
.feedback strong { display:block; margin-bottom:8px; }
.explanation { margin-top:12px; color:#3d4355; }
.solver-actions { display:flex; justify-content:space-between; gap:10px; margin-top:18px; }
.q-dots { display:flex; gap:6px; flex-wrap:wrap; margin-top:16px; }
.q-dot { width:32px; height:32px; border-radius:10px; border:1px solid var(--line); background:#fff; color:var(--muted); font-size:12px; font-weight:800; }
.q-dot.active { background:var(--text); color:#fff; border-color:var(--text); }
.q-dot.done { border-color:#91d6bd; background:#f0fbf7; color:#126f4f; }
.q-dot.correct { background:#16885f; border-color:#16885f; color:#fff; }

.table-card { overflow:auto; padding:0; }
table { width:100%; border-collapse:collapse; min-width:760px; }
th, td { padding:14px 16px; text-align:left; border-bottom:1px solid var(--line); font-size:14px; vertical-align:middle; }
th { color:var(--muted); font-size:12px; text-transform:uppercase; letter-spacing:.04em; background:#fafbfe; position:sticky; top:0; }
.badge { display:inline-flex; align-items:center; padding:6px 9px; border-radius:999px; background:#f1f2f8; color:#5d6274; font-size:12px; font-weight:800; }
.badge.admin { background:#efeefe; color:#5b5bd6; }
.mini-progress { width:130px; }
.mini-progress .progress { height:7px; }
.mini-progress small { display:block; margin-top:5px; color:var(--muted); }

.admin-grid { display:grid; grid-template-columns: 1.1fr .9fr; gap:18px; }
.dropzone { border:2px dashed #d8dbeb; border-radius:20px; padding:28px; text-align:center; background:#fbfcff; }
.dropzone input { margin-top:14px; }
.codebox { padding:16px; border-radius:16px; background:#171821; color:#d9dcf5; overflow:auto; font:12px/1.6 ui-monospace, SFMono-Regular, Menlo, monospace; white-space:pre; }
.hint { color:var(--muted); font-size:13px; line-height:1.6; }
.empty { text-align:center; padding:48px 22px; color:var(--muted); }
.notice { padding:14px 16px; border-radius:16px; background:#fff7df; border:1px solid #f0dfaa; color:#6d5718; font-size:13px; line-height:1.55; margin-bottom:18px; }

.toast { position:fixed; right:20px; bottom:20px; z-index:100; max-width:min(420px,calc(100vw - 40px)); padding:13px 16px; border-radius:14px; background:#171821; color:#fff; box-shadow:0 14px 40px rgba(0,0,0,.2); }

@media (max-width: 900px) {
  .grid { grid-template-columns: 1fr 1fr; }
  .admin-grid { grid-template-columns:1fr; }
  .auth-wrap { grid-template-columns:1fr; }
  .auth-hero { min-height: 360px; padding: 38px 28px; }
  .auth-panel { padding: 24px 18px 40px; }
}
@media (max-width: 640px) {
  .topbar { padding: 11px 14px; }
  .brand-copy small { display:none; }
  .userpill { display:none; }
  .app-body { padding: 22px 14px 44px; }
  .grid { grid-template-columns:1fr; }
  .page-head { align-items:flex-start; flex-direction:column; }
  .auth-hero { min-height:300px; }
  .auth-hero h1 { font-size:44px; }
  .auth-card { padding:22px; border-radius:24px; }
  .answer-row { flex-direction:column; }
  .answer-row .btn { width:100%; }
  .solver-actions { flex-wrap:wrap; }
  .solver-actions .btn { flex:1; min-width:130px; }
  .formula { min-height:88px; font-size:24px; }
  .demo-actions { grid-template-columns:1fr; }
}


/* v2: admin dashboard + homework completion */
.nav-link {
  border: 1px solid var(--line); background: rgba(255,255,255,.8); color: var(--muted);
  border-radius: 14px; padding: 10px 14px; font-weight: 700; text-decoration: none;
  display: inline-flex; align-items: center;
}
.nav-link:hover { color: var(--text); border-color: #cfd2df; }
.student-view-note {
  display:flex; align-items:center; justify-content:space-between; gap:14px; flex-wrap:wrap;
  padding:14px 16px; margin-bottom:18px; border-radius:18px; background:#f0efff; border:1px solid #e1dfff; color:#4f50a8;
}
.hw-card-head { display:flex; align-items:flex-start; justify-content:space-between; gap:12px; }
.badge.completed { background:#e9f8f1; color:#147253; }
.solver-top-actions { display:flex; align-items:center; gap:10px; flex-wrap:wrap; justify-content:flex-end; }
.btn.finish { background:#fff7e8; color:#8b5b05; border:1px solid #f2d69e; }
.btn.finish:hover { background:#ffefcc; }
.result-wrap { max-width:760px; margin:36px auto 0; }
.result-card { text-align:center; padding:clamp(28px,5vw,48px); }
.result-icon {
  width:74px; height:74px; border-radius:24px; display:grid; place-items:center; margin:0 auto 16px;
  background:#e9f8f1; color:#16885f; font-size:34px; font-weight:900;
}
.result-card h1 { margin:14px 0 8px; font-size:clamp(30px,6vw,46px); letter-spacing:-.04em; }
.result-card > p { color:var(--muted); line-height:1.6; }
.result-stats { display:grid; grid-template-columns:repeat(3,1fr); gap:12px; margin:28px 0; }
.result-stats > div { background:#f8f9fc; border:1px solid var(--line); border-radius:18px; padding:18px 12px; }
.result-stats strong { display:block; font-size:30px; letter-spacing:-.04em; }
.result-stats span { display:block; font-weight:800; margin-top:4px; }
.result-stats small { display:block; color:var(--muted); margin-top:5px; }
.result-actions { display:flex; justify-content:center; gap:10px; flex-wrap:wrap; }
.eyebrow { display:block; color:var(--primary); font-size:12px; letter-spacing:.12em; font-weight:900; margin-bottom:8px; }
.admin-overview { display:grid; gap:18px; }
.metric-grid { display:grid; grid-template-columns:repeat(4,minmax(0,1fr)); gap:16px; }
.metric-card span { color:var(--muted); font-size:13px; font-weight:700; }
.metric-card strong { display:block; font-size:36px; margin:10px 0 3px; letter-spacing:-.05em; }
.metric-card small { color:var(--muted); }
.admin-actions-grid { display:grid; grid-template-columns:1fr 1fr; gap:16px; }
.admin-action { border:1px solid var(--line); width:100%; text-align:left; display:flex; gap:16px; align-items:flex-start; cursor:pointer; color:var(--text); }
.admin-action:hover { transform:translateY(-2px); border-color:#ccccf3; }
.admin-action-icon { width:46px; height:46px; flex:0 0 46px; display:grid; place-items:center; border-radius:15px; background:#f0efff; color:var(--primary); font-weight:900; }
.admin-action strong { font-size:18px; }
.admin-action p { margin:7px 0 0; color:var(--muted); line-height:1.55; }
.section-head { display:flex; align-items:center; justify-content:space-between; gap:14px; margin-bottom:8px; }
.section-head h3 { margin:0; }
.recent-list { display:grid; }
.recent-row { display:flex; align-items:center; justify-content:space-between; gap:16px; padding:14px 0; border-bottom:1px solid var(--line); }
.recent-row:last-child { border-bottom:0; }
.recent-row > div:first-child span { display:block; color:var(--muted); font-size:13px; margin-top:3px; }
.recent-row > div:last-child { text-align:right; }
.recent-row > div:last-child small { display:block; color:var(--muted); margin-top:3px; }

@media (max-width: 900px) {
  .metric-grid { grid-template-columns:1fr 1fr; }
}
@media (max-width: 640px) {
  .solver-top { align-items:flex-start; }
  .solver-top-actions { flex-direction:column; align-items:flex-end; }
  .result-stats { grid-template-columns:1fr; }
  .metric-grid, .admin-actions-grid { grid-template-columns:1fr; }
  .topbar .userbar .btn.primary { padding:9px 10px; font-size:12px; }
}

/* v3: explicit early finish */
.early-finish-box {
  margin:22px 0 4px; padding:16px; border:1px solid #f2d69e; background:#fffaf0; border-radius:18px;
  display:flex; align-items:center; justify-content:space-between; gap:16px;
}
.early-finish-box strong { display:block; margin-bottom:5px; }
.early-finish-box span { display:block; color:var(--muted); font-size:13px; line-height:1.5; }
.completion-banner {
  margin:14px 0 0; padding:14px 16px; border:1px solid #bfe8d7; background:#eefaf5; border-radius:18px;
  display:flex; align-items:center; justify-content:space-between; gap:14px;
}
.completion-banner strong, .completion-banner span { display:block; }
.completion-banner span { color:var(--muted); font-size:13px; margin-top:3px; }
@media (max-width:640px){
  .early-finish-box, .completion-banner { flex-direction:column; align-items:stretch; }
  .early-finish-box .btn, .completion-banner .btn { width:100%; }
}