:root {
  --primary: #2563eb;
  --primary-dark: #1d4ed8;
  --secondary: #7c3aed;
  --success: #059669;
  --warning: #d97706;
  --danger: #dc2626;
  --bg: #f8fafc;
  --card: #ffffff;
  --text: #1e293b;
  --text-muted: #64748b;
  --border: #e2e8f0;
  --shadow: 0 1px 3px rgba(0,0,0,0.1), 0 1px 2px rgba(0,0,0,0.06);
  --shadow-md: 0 4px 6px rgba(0,0,0,0.07), 0 2px 4px rgba(0,0,0,0.06);
}
* { box-sizing: border-box; margin: 0; padding: 0; }
body { font-family: 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', sans-serif; background: var(--bg); color: var(--text); line-height: 1.6; }

/* Header */
header { background: linear-gradient(135deg, var(--primary) 0%, var(--secondary) 100%); color: white; padding: 24px 32px; display: flex; align-items: center; justify-content: space-between; box-shadow: var(--shadow-md); }
header h1 { font-size: 1.6rem; font-weight: 700; letter-spacing: 0.5px; }
header p { font-size: 0.9rem; opacity: 0.85; margin-top: 4px; }
.progress-bar-wrap { background: rgba(255,255,255,0.2); border-radius: 8px; height: 8px; width: 200px; margin-top: 8px; }
.progress-bar { background: white; border-radius: 8px; height: 8px; transition: width 0.4s; }

/* Nav */
nav { background: var(--card); border-bottom: 1px solid var(--border); display: flex; gap: 0; overflow-x: auto; }
nav button { padding: 14px 24px; border: none; background: none; cursor: pointer; font-size: 0.95rem; color: var(--text-muted); font-weight: 500; border-bottom: 3px solid transparent; transition: all 0.2s; white-space: nowrap; }
nav button:hover { color: var(--primary); background: #f1f5f9; }
nav button.active { color: var(--primary); border-bottom-color: var(--primary); background: #eff6ff; }

/* Layout */
.container { max-width: 1100px; margin: 0 auto; padding: 24px 16px; }
.page { display: none; }
.page.active { display: block; }

/* Cards */
.card { background: var(--card); border-radius: 12px; padding: 24px; box-shadow: var(--shadow); border: 1px solid var(--border); margin-bottom: 20px; }
.card h2 { font-size: 1.2rem; font-weight: 700; margin-bottom: 16px; color: var(--text); display: flex; align-items: center; gap: 8px; }
.card h3 { font-size: 1rem; font-weight: 600; margin: 16px 0 8px; color: var(--primary); }

/* Grid */
.grid-2 { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
.grid-3 { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; }
.grid-4 { display: grid; grid-template-columns: repeat(4, 1fr); gap: 16px; }
@media (max-width: 768px) { .grid-2, .grid-3, .grid-4 { grid-template-columns: 1fr; } }

/* Stat cards */
.stat-card { background: var(--card); border-radius: 12px; padding: 20px; text-align: center; box-shadow: var(--shadow); border: 1px solid var(--border); }
.stat-card .icon { font-size: 2rem; margin-bottom: 8px; }
.stat-card .value { font-size: 1.8rem; font-weight: 700; color: var(--primary); }
.stat-card .label { font-size: 0.85rem; color: var(--text-muted); margin-top: 4px; }

/* Tags */
.tag { display: inline-block; padding: 3px 10px; border-radius: 20px; font-size: 0.78rem; font-weight: 600; margin: 2px; }
.tag-blue { background: #dbeafe; color: #1d4ed8; }
.tag-green { background: #d1fae5; color: #065f46; }
.tag-orange { background: #fed7aa; color: #92400e; }
.tag-purple { background: #ede9fe; color: #5b21b6; }
.tag-red { background: #fee2e2; color: #991b1b; }

/* Table */
table { width: 100%; border-collapse: collapse; font-size: 0.9rem; }
th { background: #f1f5f9; padding: 10px 14px; text-align: left; font-weight: 600; color: var(--text-muted); border-bottom: 2px solid var(--border); }
td { padding: 10px 14px; border-bottom: 1px solid var(--border); }
tr:hover td { background: #f8fafc; }

/* Quiz */
.quiz-question { background: #f8fafc; border-radius: 10px; padding: 20px; margin-bottom: 16px; border-left: 4px solid var(--primary); }
.quiz-question p { font-weight: 600; margin-bottom: 12px; }
.quiz-options label { display: flex; align-items: flex-start; gap: 10px; padding: 10px 14px; border-radius: 8px; cursor: pointer; margin-bottom: 6px; border: 1px solid var(--border); transition: all 0.15s; }
.quiz-options label:hover { background: #eff6ff; border-color: var(--primary); }
.quiz-options input[type=radio] { margin-top: 3px; accent-color: var(--primary); }
.quiz-options label.correct { background: #d1fae5; border-color: var(--success); }
.quiz-options label.wrong { background: #fee2e2; border-color: var(--danger); }
.quiz-explanation { background: #fffbeb; border: 1px solid #fcd34d; border-radius: 8px; padding: 12px 16px; margin-top: 10px; font-size: 0.88rem; display: none; }
.quiz-explanation.show { display: block; }

/* Buttons */
.btn { padding: 10px 20px; border-radius: 8px; border: none; cursor: pointer; font-size: 0.9rem; font-weight: 600; transition: all 0.2s; }
.btn-primary { background: var(--primary); color: white; }
.btn-primary:hover { background: var(--primary-dark); }
.btn-success { background: var(--success); color: white; }
.btn-outline { background: transparent; border: 2px solid var(--primary); color: var(--primary); }
.btn-outline:hover { background: var(--primary); color: white; }
.btn-sm { padding: 6px 14px; font-size: 0.82rem; }

/* Vocab / Flashcard */
.flashcard { background: var(--card); border-radius: 10px; padding: 20px; border: 1px solid var(--border); box-shadow: var(--shadow); cursor: pointer; transition: transform 0.15s; }
.flashcard:hover { transform: translateY(-2px); box-shadow: var(--shadow-md); }
.vocab-card { background: var(--card); border-radius: 10px; padding: 20px; border: 1px solid var(--border); box-shadow: var(--shadow); cursor: pointer; transition: transform 0.15s; }
.vocab-card:hover { transform: translateY(-2px); box-shadow: var(--shadow-md); }
.vocab-card .word { font-size: 1.3rem; font-weight: 700; color: var(--primary); }
.vocab-card .phonetic { color: var(--text-muted); font-size: 0.85rem; margin: 4px 0; }
.vocab-card .meaning { font-size: 0.9rem; margin-top: 8px; }
.vocab-card .example { font-size: 0.82rem; color: var(--text-muted); margin-top: 6px; font-style: italic; border-top: 1px solid var(--border); padding-top: 8px; }
.vocab-card .meaning, .vocab-card .example { display: none; }
.vocab-card.revealed .meaning, .vocab-card.revealed .example { display: block; }

/* Writing */
.writing-prompt { background: linear-gradient(135deg, #eff6ff, #f5f3ff); border-radius: 10px; padding: 20px; border: 1px solid #c7d2fe; margin-bottom: 16px; }
.writing-prompt h4 { color: var(--primary); font-size: 1rem; margin-bottom: 8px; }
textarea { width: 100%; border: 1px solid var(--border); border-radius: 8px; padding: 12px; font-size: 0.9rem; font-family: inherit; resize: vertical; min-height: 160px; outline: none; transition: border-color 0.2s; }
textarea:focus { border-color: var(--primary); box-shadow: 0 0 0 3px rgba(37,99,235,0.1); }
.word-count { font-size: 0.8rem; color: var(--text-muted); text-align: right; margin-top: 4px; }

/* Translation */
.trans-original { background: #f0fdf4; border-left: 4px solid var(--success); padding: 16px; border-radius: 0 8px 8px 0; margin-bottom: 12px; font-size: 0.95rem; line-height: 1.8; }
.trans-reference { background: #eff6ff; border-left: 4px solid var(--primary); padding: 16px; border-radius: 0 8px 8px 0; font-size: 0.9rem; line-height: 1.8; display: none; }
.trans-reference.show { display: block; }

/* Outline */
.outline-section { border-left: 3px solid var(--primary); padding-left: 16px; margin-bottom: 20px; }
.outline-section h3 { color: var(--primary); font-size: 1rem; margin-bottom: 8px; }
.outline-item { padding: 6px 0; border-bottom: 1px dashed var(--border); font-size: 0.9rem; display: flex; align-items: flex-start; gap: 8px; }
.outline-item::before { content: "▸"; color: var(--primary); flex-shrink: 0; }
.priority-high { color: var(--danger); font-weight: 600; }
.priority-mid { color: var(--warning); font-weight: 600; }

/* Accordion */
.accordion { border: 1px solid var(--border); border-radius: 10px; overflow: hidden; margin-bottom: 12px; }
.accordion-header { padding: 14px 18px; background: #f8fafc; cursor: pointer; font-weight: 600; display: flex; justify-content: space-between; align-items: center; }
.accordion-header:hover { background: #f1f5f9; }
.accordion-body { padding: 16px 18px; display: none; border-top: 1px solid var(--border); font-size: 0.9rem; line-height: 1.8; }
.accordion-body.open { display: block; }
.accordion-arrow { transition: transform 0.2s; }
.accordion-arrow.open { transform: rotate(180deg); }

/* Misc */
.divider { border: none; border-top: 1px solid var(--border); margin: 20px 0; }
.text-muted { color: var(--text-muted); font-size: 0.88rem; }
.mt-8 { margin-top: 8px; }
.mt-16 { margin-top: 16px; }
.flex { display: flex; align-items: center; gap: 8px; }
.flex-between { display: flex; justify-content: space-between; align-items: center; }
.badge { display: inline-block; padding: 2px 8px; border-radius: 12px; font-size: 0.75rem; font-weight: 700; }
.badge-blue { background: #dbeafe; color: #1e40af; }
.badge-green { background: #d1fae5; color: #065f46; }
.badge-red { background: #fee2e2; color: #991b1b; }
ul.styled { padding-left: 20px; }
ul.styled li { margin-bottom: 6px; font-size: 0.9rem; }
.highlight { background: #fef9c3; padding: 2px 6px; border-radius: 4px; }

/* Mock test tabs */
.mock-tabs { display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 20px; }
.mock-tab { padding: 8px 18px; border-radius: 20px; border: 2px solid var(--border); background: var(--card); cursor: pointer; font-size: 0.88rem; font-weight: 600; color: var(--text-muted); transition: all 0.2s; }
.mock-tab:hover { border-color: var(--primary); color: var(--primary); }
.mock-tab.active { background: var(--primary); color: white; border-color: var(--primary); }
.mock-set-list { display: grid; grid-template-columns: repeat(5, 1fr); gap: 10px; margin-bottom: 20px; }
@media (max-width: 768px) { .mock-set-list { grid-template-columns: repeat(2, 1fr); } }
.mock-set-btn { padding: 12px; border-radius: 10px; border: 2px solid var(--border); background: var(--card); cursor: pointer; text-align: center; font-weight: 600; font-size: 0.88rem; transition: all 0.2s; }
.mock-set-btn:hover { border-color: var(--primary); color: var(--primary); background: #eff6ff; }
.mock-set-btn.active { border-color: var(--primary); background: var(--primary); color: white; }

/* Vocab group (collapsible) */
.vocab-group { border: 1px solid var(--border); border-radius: 10px; overflow: hidden; }
.vocab-group-header {
  display: flex; align-items: center; gap: 10px;
  padding: 11px 16px; cursor: pointer; user-select: none;
  background: #f8fafc; font-weight: 700; font-size: 0.92rem;
  transition: background 0.15s;
}
.vocab-group-header:hover { background: #eff6ff; }
.vocab-arrow { font-size: 0.75rem; color: var(--primary); margin-left: auto; flex-shrink: 0; }
.vocab-group-note { font-size: 0.75rem; color: var(--text-muted); font-weight: 400; }
.vocab-group-body { display: none; padding: 12px 16px; overflow-x: auto; }
.vocab-group-body table { width: 100%; font-size: 0.85rem; }
.vocab-group-body table th { background: #f1f5f9; padding: 6px 10px; text-align: left; font-size: 0.8rem; }
.vocab-group-body table td { padding: 5px 10px; border-bottom: 1px solid #f1f5f9; }
.vocab-group-body table tr:last-child td { border-bottom: none; }

/* Floating exam timer */
#exam-timer-bar {
  position: fixed; top: 0; left: 0; right: 0; z-index: 1000;
  background: var(--primary); color: white;
  display: none; align-items: center; justify-content: center; gap: 16px;
  padding: 8px 20px; font-weight: 700; font-size: 1rem;
  box-shadow: 0 2px 8px rgba(0,0,0,0.18);
}
#exam-timer-bar.warning { background: #d97706; }
#exam-timer-bar.danger { background: var(--danger); }
#exam-timer-bar.show { display: flex; }
#exam-timer-display { font-size: 1.25rem; font-variant-numeric: tabular-nums; letter-spacing: 2px; }
#exam-timer-bar button { background: rgba(255,255,255,0.2); border: 1px solid rgba(255,255,255,0.4); color: white; border-radius: 6px; padding: 3px 12px; cursor: pointer; font-size: 0.82rem; }
#exam-timer-bar button:hover { background: rgba(255,255,255,0.35); }

/* Score history */
.score-bar-row { display: flex; align-items: center; gap: 10px; margin-bottom: 8px; font-size: 0.85rem; }
.score-bar-label { width: 130px; flex-shrink: 0; color: var(--text-muted); white-space: nowrap; overflow: hidden; text-overflow: ellipsis; font-size: 0.8rem; }
.score-bar-track { flex: 1; background: #f1f5f9; border-radius: 4px; height: 16px; overflow: hidden; }
.score-bar-fill { height: 100%; border-radius: 4px; transition: width 0.5s; }
.score-bar-fill.good { background: var(--success); }
.score-bar-fill.ok { background: var(--warning); }
.score-bar-fill.bad { background: var(--danger); }
.score-bar-pct { width: 38px; text-align: right; font-weight: 700; flex-shrink: 0; font-size: 0.82rem; }

/* Wrong answer notebook */
.wrong-item { background: #fff7ed; border: 1px solid #fed7aa; border-radius: 8px; padding: 14px 16px; margin-bottom: 10px; }
.wrong-item .wrong-q { font-weight: 600; margin-bottom: 8px; font-size: 0.92rem; }
.wrong-item .wrong-ans { font-size: 0.85rem; color: var(--success); font-weight: 600; margin-bottom: 4px; }
.wrong-item .wrong-exp { font-size: 0.83rem; color: var(--text-muted); line-height: 1.5; }
.wrong-item .wrong-src { font-size: 0.78rem; color: #9ca3af; margin-top: 6px; }

/* Home quick-nav buttons */
.home-nav-btn {
  background: var(--card);
  border: 2px solid var(--border);
  border-radius: 10px;
  color: var(--text-muted);
  cursor: pointer;
  font-size: 0.88rem;
  transition: all 0.18s;
  text-align: center;
}
.home-nav-btn:hover {
  border-color: var(--primary);
  color: var(--primary);
  background: #eff6ff;
}
.home-nav-btn.active {
  border-color: var(--primary);
  background: var(--primary);
  color: white;
}