/* ============================================================
   英语语法闪卡 — 主样式表
   ============================================================ */

/* --- CSS Variables (Light Mode) --- */
:root {
  --bg: #f8f9fa;
  --bg-card: #ffffff;
  --bg-card-back: #fefefe;
  --text: #1a1a2e;
  --text-secondary: #555;
  --text-muted: #888;
  --border: #dee2e6;
  --border-card: #d0d5dd;
  --accent: #2563eb;
  --accent-hover: #1d4ed8;
  --accent-light: #eff6ff;
  --success: #16a34a;
  --success-light: #f0fdf4;
  --warning: #f59e0b;
  --warning-light: #fffbeb;
  --error: #dc2626;
  --error-light: #fef2f2;
  --tag-bg: #e2e8f0;
  --tag-text: #475569;
  --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
  --shadow: 0 1px 3px rgba(0,0,0,0.1), 0 1px 2px rgba(0,0,0,0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.1), 0 4px 6px -4px rgba(0,0,0,0.1);
  --shadow-xl: 0 20px 25px -5px rgba(0,0,0,0.1), 0 8px 10px -6px rgba(0,0,0,0.1);
  --radius: 12px;
  --radius-sm: 8px;
  --radius-xs: 4px;
  --transition: 0.2s ease;
  --font-mono: 'SF Mono', 'Cascadia Code', 'Consolas', 'Monaco', monospace;
  --font-sans: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  --max-width: 900px;
  --header-height: 60px;
}

/* --- Dark Mode --- */
[data-theme="dark"] {
  --bg: #0f172a;
  --bg-card: #1e293b;
  --bg-card-back: #1a2332;
  --text: #e2e8f0;
  --text-secondary: #94a3b8;
  --text-muted: #64748b;
  --border: #334155;
  --border-card: #475569;
  --accent: #3b82f6;
  --accent-hover: #60a5fa;
  --accent-light: #1e3a5f;
  --success: #22c55e;
  --success-light: #052e16;
  --warning: #fbbf24;
  --warning-light: #451a03;
  --error: #ef4444;
  --error-light: #450a0a;
  --tag-bg: #334155;
  --tag-text: #cbd5e1;
  --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
  --shadow: 0 1px 3px rgba(0,0,0,0.4), 0 1px 2px rgba(0,0,0,0.3);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.4), 0 4px 6px -4px rgba(0,0,0,0.3);
  --shadow-xl: 0 20px 25px -5px rgba(0,0,0,0.4), 0 8px 10px -6px rgba(0,0,0,0.3);
}

/* --- Reset & Base --- */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 16px;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-sans);
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  min-height: 100vh;
  transition: background var(--transition), color var(--transition);
}

/* --- Header --- */
.header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--bg-card);
  border-bottom: 1px solid var(--border);
  padding: 0 1.5rem;
  height: var(--header-height);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  box-shadow: var(--shadow-sm);
  transition: background var(--transition), border var(--transition);
}

.header-title {
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--text);
  white-space: nowrap;
  letter-spacing: -0.01em;
}

.header-controls {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

/* --- Buttons --- */
.btn {
  display: inline-flex;
  align-items: center;
  gap: 0.35rem;
  padding: 0.4rem 0.85rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg-card);
  color: var(--text);
  font-size: 0.85rem;
  font-family: var(--font-sans);
  cursor: pointer;
  transition: all var(--transition);
  white-space: nowrap;
  text-decoration: none;
  line-height: 1.4;
}
.btn:hover {
  background: var(--accent-light);
  border-color: var(--accent);
  color: var(--accent);
}
.btn:active {
  transform: scale(0.97);
}
.btn--primary {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}
.btn--primary:hover {
  background: var(--accent-hover);
  border-color: var(--accent-hover);
  color: #fff;
}
.btn--small {
  padding: 0.25rem 0.6rem;
  font-size: 0.78rem;
}
.btn--icon {
  width: 36px;
  height: 36px;
  padding: 0;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  font-size: 1.1rem;
}
.btn.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

/* --- Main Layout --- */
.main {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 1.5rem;
}

/* --- Stats Bar --- */
.stats-bar {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 1.5rem;
  margin-bottom: 1.25rem;
  flex-wrap: wrap;
  font-size: 0.85rem;
  color: var(--text-secondary);
}
.stat-item {
  display: flex;
  align-items: center;
  gap: 0.3rem;
}
.stat-value {
  font-weight: 700;
  color: var(--accent);
  font-size: 1rem;
}

/* --- Filter Toolbar --- */
.filter-bar {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-bottom: 1.5rem;
  align-items: center;
}

.filter-group {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
}

.filter-chip {
  padding: 0.3rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: 20px;
  background: var(--bg-card);
  color: var(--text-secondary);
  font-size: 0.8rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-sans);
  white-space: nowrap;
}
.filter-chip:hover {
  border-color: var(--accent);
  color: var(--accent);
}
.filter-chip.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

.filter-divider {
  width: 1px;
  height: 20px;
  background: var(--border);
  margin: 0 0.25rem;
}

.search-input {
  padding: 0.35rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: 20px;
  background: var(--bg-card);
  color: var(--text);
  font-size: 0.85rem;
  font-family: var(--font-sans);
  width: 180px;
  transition: all var(--transition);
}
.search-input:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.15);
  width: 240px;
}
.search-input::placeholder {
  color: var(--text-muted);
}

/* --- Flashcard --- */
.card-container {
  perspective: 1200px;
  margin-bottom: 1.5rem;
}

.card {
  position: relative;
  width: 100%;
  min-height: 320px;
  cursor: pointer;
  transition: transform 0.15s ease;
}

.card-inner {
  position: relative;
  width: 100%;
  min-height: 320px;
  transition: transform 0.5s cubic-bezier(0.4, 0, 0.2, 1);
  transform-style: preserve-3d;
}

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

.card-face {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  min-height: 320px;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  border-radius: var(--radius);
  border: 1px solid var(--border-card);
  background: var(--bg-card);
  box-shadow: var(--shadow-lg);
  padding: 2rem 2rem 3rem;
  transition: background var(--transition), border var(--transition), box-shadow var(--transition);
}

.card-back {
  transform: rotateY(180deg);
  overflow-y: auto;
  max-height: 70vh;
  cursor: default;
}

/* --- Card Front --- */
.card-front {
  display: flex;
  flex-direction: column;
}

.card-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-bottom: 1.25rem;
}

.card-badge {
  display: inline-block;
  padding: 0.15rem 0.55rem;
  border-radius: 12px;
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.02em;
}
.card-badge--part {
  background: var(--accent-light);
  color: var(--accent);
}
.card-badge--cat {
  background: var(--tag-bg);
  color: var(--tag-text);
}
.card-badge--diff {
  background: var(--warning-light);
  color: var(--warning);
}

.card-question-type {
  font-size: 0.75rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.75rem;
}

.card-question-text {
  font-size: 1.15rem;
  line-height: 1.8;
  color: var(--text);
  flex: 1;
}
.card-question-text .blank {
  display: inline-block;
  min-width: 80px;
  border-bottom: 2px dashed var(--accent);
  margin: 0 0.3rem;
  padding: 0 0.5rem;
  color: var(--accent);
  font-weight: 600;
}

.card-options {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
  margin-top: 1.25rem;
}
.card-option {
  padding: 0.6rem 1rem;
  border: 2px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--bg-card);
  color: var(--text);
  font-size: 0.95rem;
  cursor: pointer;
  transition: all var(--transition);
  text-align: left;
  font-family: var(--font-sans);
}
.card-option:hover {
  border-color: var(--accent);
  background: var(--accent-light);
}

.card-hint {
  margin-top: 1.25rem;
  padding: 0.75rem 1rem;
  background: var(--warning-light);
  border-radius: var(--radius-sm);
  font-size: 0.85rem;
  color: var(--text-secondary);
  border-left: 3px solid var(--warning);
  display: none;
}
.card-hint.visible {
  display: block;
}

.card-action-bar {
  position: absolute;
  bottom: 0.75rem;
  right: 1rem;
  display: flex;
  gap: 0.5rem;
}
.card-action-btn {
  font-size: 0.75rem;
  padding: 0.25rem 0.65rem;
  border-radius: 12px;
  border: 1px solid var(--border);
  background: var(--bg-card);
  color: var(--text-muted);
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-sans);
}
.card-action-btn:hover {
  color: var(--accent);
  border-color: var(--accent);
}

.card-flip-hint {
  position: absolute;
  bottom: 0.75rem;
  left: 1rem;
  font-size: 0.75rem;
  color: var(--text-muted);
}

/* --- Card Back (Answer) --- */
.card-back-content {
  padding-bottom: 1rem;
}

.answer-correct {
  display: inline-block;
  padding: 0.25rem 0.75rem;
  background: var(--success-light);
  color: var(--success);
  border-radius: var(--radius-xs);
  font-weight: 700;
  font-size: 1rem;
  margin-bottom: 1rem;
}

.answer-explanation {
  margin: 1rem 0;
  line-height: 1.8;
  color: var(--text);
}
.answer-explanation strong {
  color: var(--accent);
}
.answer-explanation code {
  background: var(--tag-bg);
  padding: 0.1rem 0.35rem;
  border-radius: 3px;
  font-family: var(--font-mono);
  font-size: 0.9em;
  color: var(--error);
}

.answer-section {
  margin-top: 1.25rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
}
.answer-section-title {
  font-size: 0.8rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
}

.example-list {
  list-style: none;
  padding: 0;
}
.example-item {
  padding: 0.5rem 0.75rem;
  margin-bottom: 0.4rem;
  background: var(--accent-light);
  border-radius: var(--radius-xs);
  font-family: var(--font-mono);
  font-size: 0.9rem;
  line-height: 1.6;
  color: var(--text);
  border-left: 3px solid var(--accent);
}
.example-item .correct {
  color: var(--success);
  font-weight: 600;
}
.example-item .wrong {
  color: var(--error);
}

.grammar-points-list {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
}
.grammar-point-tag {
  padding: 0.2rem 0.6rem;
  background: var(--tag-bg);
  color: var(--tag-text);
  border-radius: 12px;
  font-size: 0.78rem;
}

.related-cards-list {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
}
.related-card-link {
  padding: 0.2rem 0.6rem;
  background: var(--accent-light);
  color: var(--accent);
  border-radius: 12px;
  font-size: 0.78rem;
  cursor: pointer;
  border: 1px solid transparent;
  transition: all var(--transition);
  font-family: var(--font-sans);
}
.related-card-link:hover {
  border-color: var(--accent);
  background: var(--accent);
  color: #fff;
}

/* --- Sentence Analysis (Part 2) --- */
.analysis-block {
  margin: 0.75rem 0;
  padding: 1rem;
  background: var(--bg);
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
}
.analysis-label {
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.5rem;
}
.analysis-content {
  font-family: var(--font-mono);
  font-size: 0.88rem;
  line-height: 1.8;
  color: var(--text);
  white-space: pre-wrap;
}

.parse-tree {
  font-family: var(--font-mono);
  font-size: 0.85rem;
  line-height: 1.8;
  padding: 0.75rem 1rem;
  background: var(--bg);
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  overflow-x: auto;
  white-space: pre;
}
.parse-tree .clause { color: var(--accent); font-weight: 600; }
.parse-tree .subject { color: var(--success); }
.parse-tree .verb { color: var(--error); }
.parse-tree .object { color: var(--warning); }
.parse-tree .modifier { color: var(--text-muted); }

.translation-block {
  margin-top: 1rem;
  padding: 1rem;
  background: var(--warning-light);
  border-radius: var(--radius-sm);
  border-left: 3px solid var(--warning);
}

/* --- Principles Card (Part 3) --- */
.principle-question {
  font-size: 1.1rem;
  line-height: 1.7;
  margin-bottom: 1rem;
}
.principle-insight {
  padding: 1rem;
  background: var(--accent-light);
  border-radius: var(--radius-sm);
  border-left: 4px solid var(--accent);
  margin: 1rem 0;
  line-height: 1.7;
}

.compare-table {
  width: 100%;
  border-collapse: collapse;
  margin: 0.75rem 0;
  font-size: 0.88rem;
}
.compare-table th {
  background: var(--accent-light);
  color: var(--accent);
  padding: 0.5rem 0.75rem;
  text-align: left;
  font-weight: 600;
  border: 1px solid var(--border);
}
.compare-table td {
  padding: 0.5rem 0.75rem;
  border: 1px solid var(--border);
}

/* --- Navigation --- */
.nav-bar {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 0.75rem;
  margin-top: 1.5rem;
  flex-wrap: wrap;
}

.nav-btn {
  padding: 0.5rem 1rem;
  font-size: 0.9rem;
}

.card-counter {
  font-size: 0.85rem;
  color: var(--text-muted);
  min-width: 60px;
  text-align: center;
}

/* --- Rating Buttons --- */
.rating-bar {
  display: flex;
  justify-content: center;
  gap: 0.5rem;
  margin-top: 1rem;
  flex-wrap: wrap;
}
.rating-btn {
  padding: 0.45rem 0.9rem;
  border-radius: 20px;
  border: 1px solid var(--border);
  background: var(--bg-card);
  color: var(--text-secondary);
  font-size: 0.8rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-sans);
}
.rating-btn:hover {
  transform: translateY(-1px);
}
.rating-btn--again { border-color: var(--error); color: var(--error); }
.rating-btn--again:hover { background: var(--error-light); }
.rating-btn--hard { border-color: var(--warning); color: var(--warning); }
.rating-btn--hard:hover { background: var(--warning-light); }
.rating-btn--good { border-color: var(--accent); color: var(--accent); }
.rating-btn--good:hover { background: var(--accent-light); }
.rating-btn--easy { border-color: var(--success); color: var(--success); }
.rating-btn--easy:hover { background: var(--success-light); }

/* --- Progress Ring --- */
.progress-ring {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}
.progress-bar-bg {
  width: 100px;
  height: 6px;
  background: var(--border);
  border-radius: 3px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  background: var(--accent);
  border-radius: 3px;
  transition: width 0.3s ease;
}

/* --- Empty State --- */
.empty-state {
  text-align: center;
  padding: 3rem 1rem;
  color: var(--text-muted);
}
.empty-state-icon {
  font-size: 3rem;
  margin-bottom: 1rem;
}
.empty-state-text {
  font-size: 1rem;
}

/* --- Toast --- */
.toast {
  position: fixed;
  bottom: 2rem;
  left: 50%;
  transform: translateX(-50%);
  padding: 0.6rem 1.25rem;
  background: var(--text);
  color: var(--bg);
  border-radius: 20px;
  font-size: 0.85rem;
  box-shadow: var(--shadow-lg);
  opacity: 0;
  transition: opacity 0.3s ease;
  pointer-events: none;
  z-index: 1001;
}
.toast.visible {
  opacity: 1;
}

/* --- Modal --- */
.modal-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.5);
  z-index: 200;
  display: flex;
  align-items: center;
  justify-content: center;
  animation: fadeIn 0.2s ease;
}
.modal {
  background: var(--bg-card);
  border-radius: var(--radius);
  padding: 2rem;
  max-width: 500px;
  width: 90%;
  box-shadow: var(--shadow-xl);
  max-height: 80vh;
  overflow-y: auto;
}
.modal h3 {
  margin-bottom: 1rem;
}
.modal-close {
  float: right;
  background: none;
  border: none;
  font-size: 1.25rem;
  cursor: pointer;
  color: var(--text-muted);
  padding: 0.25rem;
}

/* --- Animations --- */
@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}
@keyframes slideUp {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}
.card-face {
  animation: slideUp 0.3s ease;
}

/* --- Responsive --- */
@media (max-width: 768px) {
  .header {
    padding: 0 0.75rem;
  }
  .header-title {
    font-size: 0.95rem;
  }
  .main {
    padding: 1rem 0.75rem;
  }
  .card-face {
    padding: 1.25rem 1.25rem 2.5rem;
    min-height: 280px;
  }
  .card-question-text {
    font-size: 1rem;
  }
  .search-input {
    width: 120px;
  }
  .search-input:focus {
    width: 160px;
  }
  .filter-bar {
    gap: 0.35rem;
  }
  .filter-chip {
    font-size: 0.75rem;
    padding: 0.25rem 0.6rem;
  }
  .rating-btn {
    padding: 0.35rem 0.7rem;
    font-size: 0.75rem;
  }
}

@media (max-width: 480px) {
  .header-title {
    font-size: 0.85rem;
  }
  .btn {
    font-size: 0.75rem;
    padding: 0.3rem 0.6rem;
  }
  .card-face {
    padding: 1rem 1rem 2rem;
  }
  .stats-bar {
    gap: 0.75rem;
  }
}

/* --- Print --- */
@media print {
  .header, .nav-bar, .rating-bar, .filter-bar, .stats-bar {
    display: none;
  }
  .card-inner {
    transform: none !important;
  }
  .card-face {
    position: static;
    box-shadow: none;
    break-inside: avoid;
  }
  .card-back {
    transform: none;
    display: block !important;
  }
}

/* ============================================================
   Auth Modal & UI
   ============================================================ */

/* Modal backdrop */
.auth-modal {
  display: none;
  position: fixed;
  inset: 0;
  z-index: 1000;
  align-items: center;
  justify-content: center;
}
.auth-modal.visible {
  display: flex;
}
.auth-modal-backdrop {
  position: absolute;
  inset: 0;
  background: rgba(0,0,0,0.5);
  backdrop-filter: blur(4px);
}
.auth-modal-content {
  position: relative;
  background: var(--bg-card);
  border-radius: var(--radius);
  box-shadow: var(--shadow-xl);
  padding: 2rem;
  width: 100%;
  max-width: 400px;
  z-index: 1;
  animation: modalSlideIn 0.25s ease;
}
@keyframes modalSlideIn {
  from { opacity: 0; transform: translateY(-20px); }
  to   { opacity: 1; transform: translateY(0); }
}
.auth-modal-close {
  position: absolute;
  top: 0.75rem;
  right: 1rem;
  background: none;
  border: none;
  font-size: 1.5rem;
  cursor: pointer;
  color: var(--text-muted);
  line-height: 1;
}

/* Auth tabs */
.auth-tabs {
  display: flex;
  gap: 0;
  margin-bottom: 1.5rem;
  border-bottom: 2px solid var(--border);
}
.auth-tab {
  flex: 1;
  padding: 0.75rem;
  border: none;
  background: none;
  font-size: 1rem;
  cursor: pointer;
  color: var(--text-muted);
  border-bottom: 2px solid transparent;
  margin-bottom: -2px;
  transition: color var(--transition), border-color var(--transition);
}
.auth-tab.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
  font-weight: 600;
}

/* Auth form */
.auth-form {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}
.auth-form-group {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}
.auth-form-group label {
  font-size: 0.85rem;
  color: var(--text-secondary);
  font-weight: 500;
}
.auth-form-group input {
  padding: 0.65rem 0.75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: 0.95rem;
  background: var(--bg);
  color: var(--text);
  transition: border-color var(--transition);
}
.auth-form-group input:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-light);
}
.auth-submit {
  padding: 0.75rem;
  font-size: 1rem;
  font-weight: 600;
  border: none;
  border-radius: var(--radius-sm);
  background: var(--accent);
  color: #fff;
  cursor: pointer;
  transition: background var(--transition);
}
.auth-submit:hover {
  background: var(--accent-hover);
}

/* Auth error */
.auth-error {
  color: var(--error);
  font-size: 0.85rem;
  min-height: 1.2em;
}

/* Guest hint */
.auth-guest-hint {
  text-align: center;
  margin-top: 1rem;
  font-size: 0.85rem;
  color: var(--text-muted);
}
.btn-link {
  background: none;
  border: none;
  color: var(--accent);
  cursor: pointer;
  font-size: inherit;
  text-decoration: underline;
  padding: 0;
}

/* Auth area in header */
#auth-area {
  display: flex;
  align-items: center;
  gap: 8px;
}

/* Checkin button states */
#btn-checkin.checked-in {
  background: var(--success);
  color: #fff;
  border-color: var(--success);
}

/* Btn primary variant */
.btn--primary {
  background: var(--accent);
  color: #fff;
}
.btn--primary:hover {
  background: var(--accent-hover);
}