* { box-sizing: border-box; margin: 0; padding: 0; }

:root {
    --bg: #f8f9fa;
    --bg-card: #ffffff;
    --bg-hover: #e9ecef;
    --border: #dee2e6;
    --text: #212529;
    --text-dim: #6c757d;
    --primary: #4361ee;
    --primary-light: #7b8ff8;
    --success: #2d9a3d;
    --danger: #dc3545;
    --warning: #ffc107;
    --hint-1: #e3f2fd;
    --hint-2: #fff3e0;
    --hint-3: #fce4ec;
}

body { background: var(--bg); color: var(--text); font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Microsoft JhengHei", sans-serif; font-size: 16px; line-height: 1.6; }

/* Topbar */
.topbar { background: var(--bg-card); border-bottom: 1px solid var(--border); padding: 10px 20px; display: flex; align-items: center; gap: 20px; position: sticky; top: 0; z-index: 10; }
.brand { font-size: 18px; font-weight: 700; }
.nav { display: flex; gap: 12px; flex: 1; }
.nav a { color: var(--text-dim); text-decoration: none; padding: 4px 10px; border-radius: 6px; font-size: 14px; }
.nav a:hover { background: var(--bg-hover); }
.user-info { display: flex; align-items: center; gap: 10px; font-size: 14px; color: var(--text-dim); }
.btn-logout { color: var(--danger); text-decoration: none; font-size: 13px; }

main { padding: 20px; max-width: 900px; margin: 0 auto; }

/* Login */
.login-page { min-height: 100vh; display: flex; align-items: center; justify-content: center; background: linear-gradient(135deg, #667eea 0%, #764ba2 100%); }
.login-card { background: white; border-radius: 16px; padding: 40px; width: 360px; text-align: center; box-shadow: 0 20px 60px rgba(0,0,0,0.15); }
.login-card h1 { font-size: 28px; margin-bottom: 4px; }
.login-card .subtitle { color: var(--text-dim); margin-bottom: 24px; }
.login-card label { display: block; text-align: left; margin-bottom: 14px; font-size: 14px; color: var(--text-dim); }
.login-card input { width: 100%; padding: 10px 12px; border: 1px solid var(--border); border-radius: 8px; font-size: 15px; margin-top: 4px; }
.login-card input:focus { outline: none; border-color: var(--primary); box-shadow: 0 0 0 3px rgba(67,97,238,0.1); }
.error { background: #fce4ec; color: var(--danger); padding: 8px 12px; border-radius: 6px; margin-bottom: 16px; font-size: 14px; }

/* Buttons */
.btn-primary { display: inline-block; background: var(--primary); color: white; border: none; padding: 10px 24px; border-radius: 8px; font-size: 15px; font-weight: 600; cursor: pointer; text-decoration: none; text-align: center; width: 100%; }
.btn-primary:hover { background: var(--primary-light); }
.btn-big { padding: 14px 32px; font-size: 17px; }
.btn-secondary { display: inline-block; background: var(--bg-hover); color: var(--text); border: 1px solid var(--border); padding: 10px 20px; border-radius: 8px; font-size: 14px; cursor: pointer; text-decoration: none; }
.btn-hint { display: inline-block; background: var(--warning); color: #000; border: none; padding: 10px 20px; border-radius: 8px; font-size: 14px; font-weight: 600; cursor: pointer; text-decoration: none; }

/* Filters */
.filters { display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 16px; }
.chip { padding: 6px 14px; border-radius: 20px; background: var(--bg-card); border: 1px solid var(--border); color: var(--text-dim); text-decoration: none; font-size: 13px; }
.chip:hover { background: var(--bg-hover); }
.chip.active { background: var(--primary); color: white; border-color: var(--primary); }

.empty { text-align: center; padding: 60px; color: var(--text-dim); }
.hint-text { font-size: 13px; margin-top: 8px; }

/* Question Grid */
.question-grid { display: grid; gap: 10px; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); }
.q-card { background: var(--bg-card); border: 1px solid var(--border); border-radius: 10px; padding: 14px; text-decoration: none; color: var(--text); transition: all 0.15s; border-left: 4px solid var(--border); }
.q-card:hover { border-color: var(--primary); transform: translateY(-2px); box-shadow: 0 4px 12px rgba(0,0,0,0.06); }
.q-card.done { border-left-color: var(--success); background: #f0fdf4; }
.q-card.wrong { border-left-color: var(--danger); background: #fef2f2; }
.q-header { display: flex; align-items: center; gap: 8px; margin-bottom: 6px; }
.q-chapter { font-size: 12px; color: var(--text-dim); background: var(--bg-hover); padding: 2px 8px; border-radius: 4px; }
.q-number { font-size: 14px; font-weight: 600; }
.q-badge { font-size: 11px; padding: 2px 8px; border-radius: 10px; margin-left: auto; font-weight: 600; }
.q-badge.correct { background: #dcfce7; color: var(--success); }
.q-badge.wrong { background: #fecaca; color: var(--danger); }
.q-badge.new { background: #e0e7ff; color: var(--primary); }
.q-preview { font-size: 13px; color: var(--text-dim); line-height: 1.5; }
.q-meta { font-size: 11px; color: var(--text-dim); margin-top: 6px; }

/* Question View */
.breadcrumb { display: flex; gap: 8px; align-items: center; margin-bottom: 16px; font-size: 14px; color: var(--text-dim); }
.breadcrumb a { color: var(--primary); text-decoration: none; }

.question-view { background: var(--bg-card); border: 1px solid var(--border); border-radius: 12px; padding: 24px; }
.q-title { font-size: 14px; color: var(--text-dim); margin-bottom: 12px; font-weight: 600; }
.q-content { font-size: 17px; line-height: 1.8; margin-bottom: 20px; white-space: pre-wrap; }
.q-image { margin: 16px 0; text-align: center; }
.q-image img { max-width: 100%; border-radius: 8px; border: 1px solid var(--border); }

/* Options (clickable cards) */
.options { margin-bottom: 20px; }
.option { padding: 12px 16px; margin: 8px 0; border-radius: 10px; background: var(--bg); border: 2px solid var(--border); font-size: 15px; display: flex; gap: 10px; align-items: flex-start; transition: all 0.15s; }
.option.clickable { cursor: pointer; }
.option.clickable:hover { border-color: var(--primary); background: #f0f4ff; }
.option.selected { border-color: var(--primary); background: #e0e7ff; }
.option-key { font-weight: 700; min-width: 30px; color: var(--primary); }
.selected-correct { background: #dcfce7 !important; border-color: var(--success) !important; }
.selected-wrong { background: #fecaca !important; border-color: var(--danger) !important; }
.correct-answer { background: #dcfce7; border-color: var(--success); }

/* Answer Form */
.answer-form { margin: 20px 0; }
.action-buttons { display: flex; gap: 12px; flex-wrap: wrap; align-items: stretch; }
.action-buttons .btn-primary, .action-buttons .btn-hint { flex: 1; min-width: 150px; display: flex; align-items: center; justify-content: center; padding: 14px 24px; font-size: 16px; box-sizing: border-box; }

/* Keyword highlighting in question */
.kw-highlight { color: #dc2626; font-weight: 700; text-decoration: underline; text-decoration-color: #dc2626; text-underline-offset: 3px; }

/* Results */
.result { padding: 16px; border-radius: 10px; margin: 16px 0; }
.result h3 { font-size: 18px; margin-bottom: 4px; }
.result p { font-size: 14px; color: var(--text-dim); }
.result-correct { background: #dcfce7; border: 1px solid #86efac; }
.result-wrong { background: #fecaca; border: 1px solid #fca5a5; }
.result-final { background: #fef3c7; border: 1px solid #fcd34d; }

/* Hints */
.hint { margin: 12px 0; border-radius: 10px; padding: 14px; }
.hint-1 { background: var(--hint-1); border: 1px solid #90caf9; }
.hint-2 { background: var(--hint-2); border: 1px solid #ffcc80; }
.hint-3 { background: var(--hint-3); border: 1px solid #ef9a9a; }
.hint-label { font-size: 13px; font-weight: 600; margin-bottom: 6px; }
.hint-content { font-size: 15px; line-height: 1.7; white-space: pre-wrap; }

.solution { margin: 12px 0; background: #f3e8ff; border: 1px solid #c084fc; border-radius: 10px; padding: 14px; }

.next-stage { display: flex; gap: 10px; margin-top: 16px; flex-wrap: wrap; }

/* Math content */
.math-content .katex { font-size: 1.1em; }

/* Mobile */
@media (max-width: 640px) {
    .topbar { flex-wrap: wrap; gap: 8px; padding: 10px 12px; }
    main { padding: 12px; }
    .question-view { padding: 16px; }
    .q-content { font-size: 15px; }
    .answer-select { flex-direction: column; }
}