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

:root {
  --color-primary: #1a5276;
  --color-primary-light: #2980b9;
  --color-bg: #f8f9fa;
  --color-surface: #ffffff;
  --color-text: #2c3e50;
  --color-text-light: #7f8c8d;
  --color-border: #dee2e6;
  --color-danger: #e74c3c;
  --color-warning: #f39c12;
  --color-success: #27ae60;
  --color-accent: #8e44ad;
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.08);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.1);
  --radius: 8px;
  --radius-lg: 12px;
  --font-mono: 'Consolas', 'Courier New', monospace;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  line-height: 1.6;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* === Header === */
.app-header {
  background: var(--color-primary);
  color: white;
  padding: 0.75rem 1.5rem;
  box-shadow: var(--shadow-md);
  position: sticky;
  top: 0;
  z-index: 100;
}

.header-top {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  margin-bottom: 0.5rem;
  min-width: 0;
}
.header-logo {
  width: 36px;
  height: 36px;
  max-width: 36px;
  max-height: 36px;
  flex-shrink: 0;
  object-fit: contain;
}
.header-title {
  font-size: 1.25rem;
  font-weight: 600;
  margin: 0;
  min-width: 0;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

/* === Navigation === */
.main-nav {
  display: flex;
  gap: 0.25rem;
  flex-wrap: nowrap;
}
.nav-btn {
  background: rgba(255,255,255,0.15);
  color: white;
  border: none;
  padding: 0.4rem 0.55rem;
  border-radius: var(--radius);
  font-size: 0.8rem;
  cursor: pointer;
  transition: background 0.2s;
  white-space: nowrap;
  flex: 1 1 auto;
  text-align: center;
}
.nav-btn:hover { background: rgba(255,255,255,0.25); }
.nav-btn.active { background: rgba(255,255,255,0.35); font-weight: 600; }

/* Mobile: show short label, desktop: show full label */
.nav-short { display: none; }
.nav-full { display: inline; }

/* === Main Content === */
.app-main {
  flex: 1;
  padding: 1.5rem;
  max-width: 1100px;
  width: 100%;
  margin: 0 auto;
}

/* === Cards === */
.card {
  background: var(--color-surface);
  border-radius: var(--radius-lg);
  padding: 1.25rem;
  box-shadow: var(--shadow-sm);
  margin-bottom: 1rem;
}

/* === Page Sections === */
.page-section { display: none; }
.page-section.active { display: block; }

/* === Footer === */
.app-footer {
  text-align: center;
  padding: 1rem;
  color: var(--color-text-light);
  font-size: 0.8rem;
  border-top: 1px solid var(--color-border);
}

/* === Utilities === */
.text-center { text-align: center; }
.mt-1 { margin-top: 0.5rem; }
.mt-2 { margin-top: 1rem; }
.flex-row { display: flex; gap: 0.75rem; align-items: center; flex-wrap: wrap; }
.flex-between { display: flex; justify-content: space-between; align-items: center; }

/* === Welcome === */
.welcome {
  text-align: center;
  padding: 4rem 1rem;
}
.welcome h2 { color: var(--color-primary); font-size: 1.5rem; margin-bottom: 0.5rem; }
.welcome p { color: var(--color-text-light); }

/* === Responsive === */
/* === Course Cards === */
.course-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 0.75rem;
}
.course-card {
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 1rem;
  transition: box-shadow 0.2s;
}
.course-card:hover { box-shadow: var(--shadow-md); }
.course-card-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 0.5rem;
}
.course-category {
  font-size: 0.7rem;
  text-transform: uppercase;
  color: var(--color-primary-light);
  font-weight: 600;
  letter-spacing: 0.5px;
}
.course-credits {
  font-size: 0.75rem;
  color: var(--color-text-light);
  font-weight: 500;
}
.course-name {
  font-size: 0.95rem;
  margin-bottom: 0.15rem;
  color: var(--color-text);
}
.course-name-cn {
  font-size: 0.8rem;
  color: var(--color-text-light);
  margin-bottom: 0.75rem;
}
.progress-bar-bg {
  background: var(--color-border);
  height: 6px;
  border-radius: 3px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  border-radius: 3px;
  transition: width 0.5s ease;
}
.progress-label {
  font-size: 0.75rem;
  color: var(--color-text-light);
}

/* === Buttons === */
.btn-sm {
  background: var(--color-primary-light);
  color: white;
  border: none;
  padding: 0.3rem 0.75rem;
  border-radius: var(--radius);
  font-size: 0.75rem;
  cursor: pointer;
  transition: background 0.2s;
}
.btn-sm:hover { background: var(--color-primary); }

/* === Semester Group === */
.semester-group h3 {
  font-size: 1.1rem;
  color: var(--color-primary);
}

/* === Flash Card === */
.flashcard-container {
  max-width: 480px;
  margin: 0 auto;
  perspective: 1000px;
}
.flashcard {
  width: 100%;
  height: 280px;
  position: relative;
  transform-style: preserve-3d;
  transition: transform 0.5s;
  cursor: pointer;
}
.flashcard.flipped { transform: rotateY(180deg); }
.flashcard-face {
  position: absolute;
  width: 100%;
  height: 100%;
  backface-visibility: hidden;
  border-radius: var(--radius-lg);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  text-align: center;
  box-shadow: var(--shadow-md);
}
.flashcard-front {
  background: var(--color-primary);
  color: white;
}
.flashcard-front .card-word {
  font-size: 2rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
}
.flashcard-front .card-hint {
  font-size: 0.85rem;
  opacity: 0.7;
}
.flashcard-back {
  background: var(--color-surface);
  color: var(--color-text);
  transform: rotateY(180deg);
}
.flashcard-back .card-word-cn {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-primary);
  margin-bottom: 1rem;
}
.flashcard-back .card-definition {
  font-size: 0.95rem;
  line-height: 1.5;
  color: var(--color-text);
}

/* === Rating Buttons === */
.rating-buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.5rem;
  margin-top: 1rem;
  max-width: 480px;
  margin-left: auto;
  margin-right: auto;
}
.rating-btn {
  padding: 0.6rem 0.25rem;
  border: 2px solid var(--color-border);
  border-radius: var(--radius);
  background: var(--color-surface);
  font-size: 0.8rem;
  cursor: pointer;
  text-align: center;
  transition: all 0.15s;
}
.rating-btn .rating-label { font-weight: 600; display: block; }
.rating-btn .rating-desc { font-size: 0.65rem; color: var(--color-text-light); }
.rating-btn:hover { border-color: var(--color-primary-light); }
.rating-btn.rating-again:hover { border-color: var(--color-danger); background: #fdf2f2; }
.rating-btn.rating-hard:hover { border-color: var(--color-warning); background: #fef9e7; }
.rating-btn.rating-good:hover { border-color: var(--color-primary-light); background: #eaf2f8; }
.rating-btn.rating-easy:hover { border-color: var(--color-success); background: #eafaf1; }

/* === Flash Card Detail Panel === */
.btn-detail-toggle {
  margin-top: 0.75rem;
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 0.35rem 0.75rem;
  font-size: 0.75rem;
  cursor: pointer;
  color: var(--color-text-light);
  transition: all 0.2s;
  display: flex;
  align-items: center;
  gap: 0.4rem;
}
.btn-detail-toggle:hover { border-color: var(--color-primary-light); color: var(--color-primary); }
.btn-detail-toggle .detail-toggle-arrow {
  font-size: 0.6rem;
  transition: transform 0.3s;
}
.btn-detail-toggle.active { border-color: var(--color-primary); color: var(--color-primary); background: #eaf2f8; }
.btn-detail-toggle.active .detail-toggle-arrow { transform: rotate(180deg); }

.detail-panel {
  max-width: 500px;
  margin: 0.5rem auto 0;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.4s ease;
  background: var(--color-surface);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
}
.detail-panel.visible { max-height: 600px; }
.detail-section {
  padding: 1rem 1.25rem;
  border-bottom: 1px solid var(--color-border);
}
.detail-section:last-child { border-bottom: none; }
.detail-section-title {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--color-primary-light);
  margin-bottom: 0.5rem;
  text-transform: uppercase;
  letter-spacing: 0.3px;
}
.detail-section-body {
  font-size: 0.85rem;
  line-height: 1.7;
  color: var(--color-text);
}

/* === Vocab Stats === */
.vocab-stats {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 0.75rem;
  margin-bottom: 1.5rem;
}
.stat-card {
  background: var(--color-surface);
  border-radius: var(--radius);
  padding: 1rem;
  text-align: center;
  box-shadow: var(--shadow-sm);
}
.stat-value {
  font-size: 2rem;
  font-weight: 700;
  color: var(--color-primary);
}
.stat-label {
  font-size: 0.8rem;
  color: var(--color-text-light);
  margin-top: 0.25rem;
}

/* === Knowledge Tree === */
.breadcrumb {
  font-size: 0.85rem;
  color: var(--color-text-light);
  margin-bottom: 1rem;
  cursor: pointer;
}
.breadcrumb span:hover { color: var(--color-primary); text-decoration: underline; }
.knowledge-list { display: flex; flex-direction: column; gap: 0.5rem; }
.knowledge-item {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 1rem;
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
  transition: box-shadow 0.15s;
}
.knowledge-item:hover { box-shadow: var(--shadow-md); }
.knowledge-item .ki-title { font-weight: 600; font-size: 0.95rem; }
.knowledge-item .ki-meta { font-size: 0.75rem; color: var(--color-text-light); }
.knowledge-detail { max-width: 700px; }
.knowledge-detail h3 { color: var(--color-primary); margin-bottom: 0.25rem; }
.knowledge-detail h4 { color: var(--color-text-light); font-weight: normal; margin-bottom: 1rem; }
.knowledge-section {
  background: var(--color-bg);
  border-radius: var(--radius);
  padding: 1rem;
  margin-bottom: 0.75rem;
}
.knowledge-section h5 { font-size: 0.8rem; color: var(--color-primary-light); margin-bottom: 0.5rem; text-transform: uppercase; letter-spacing: 0.5px; }
.ai-badge {
  display: inline-block;
  font-size: 0.65rem;
  background: var(--color-accent);
  color: white;
  padding: 0.15rem 0.5rem;
  border-radius: 4px;
  margin-left: 0.5rem;
}
.tag {
  display: inline-block;
  font-size: 0.7rem;
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  padding: 0.15rem 0.5rem;
  border-radius: 4px;
  margin-right: 0.25rem;
  margin-bottom: 0.25rem;
  color: var(--color-text-light);
}

/* === Quiz === */
.quiz-container { max-width: 650px; }
.quiz-question {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 1.5rem;
  margin-bottom: 1rem;
  box-shadow: var(--shadow-sm);
}
.quiz-question .qq-number {
  font-size: 0.8rem;
  color: var(--color-primary-light);
  font-weight: 600;
  margin-bottom: 0.5rem;
}
.quiz-question .qq-text { font-size: 1rem; font-weight: 500; margin-bottom: 1rem; }
.quiz-option {
  display: block;
  width: 100%;
  text-align: left;
  padding: 0.6rem 1rem;
  margin-bottom: 0.25rem;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  background: var(--color-surface);
  cursor: pointer;
  font-size: 0.9rem;
  transition: all 0.15s;
}
.quiz-option:hover { border-color: var(--color-primary-light); background: #eaf2f8; }
.quiz-option.selected { border-color: var(--color-primary); background: #d4e6f1; font-weight: 600; }
.quiz-option.correct { border-color: var(--color-success); background: #eafaf1; }
.quiz-option.wrong { border-color: var(--color-danger); background: #fdf2f2; }
.quiz-score { text-align: center; padding: 2rem; }
.quiz-score .score-number { font-size: 3rem; font-weight: 700; color: var(--color-primary); }

/* === Dashboard === */
.gpa-input-group { display: flex; gap: 0.5rem; align-items: center; margin-bottom: 0.5rem; }
.gpa-input-group label { width: 180px; font-size: 0.85rem; }
.gpa-input-group input { width: 80px; padding: 0.3rem 0.5rem; border: 1px solid var(--color-border); border-radius: var(--radius); font-size: 0.85rem; }
.gpa-result { font-size: 2rem; font-weight: 700; color: var(--color-primary); text-align: center; padding: 1rem; }
.suggestion-list { list-style: none; padding: 0; }
.suggestion-list li {
  padding: 0.6rem 1rem;
  background: var(--color-bg);
  border-radius: var(--radius);
  margin-bottom: 0.4rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.9rem;
}
.suggestion-priority {
  font-size: 0.65rem;
  font-weight: 700;
  padding: 0.15rem 0.5rem;
  border-radius: 4px;
  color: white;
  min-width: 30px;
  text-align: center;
}
.priority-high { background: var(--color-danger); }
.priority-mid { background: var(--color-warning); }
.priority-low { background: var(--color-success); }

/* === Empty / Loading states === */
.empty-state {
  text-align: center;
  padding: 3rem 1rem;
  color: var(--color-text-light);
}
.empty-state .empty-icon { font-size: 3rem; margin-bottom: 0.5rem; }
.loading { text-align: center; padding: 2rem; color: var(--color-text-light); }

@media (max-width: 480px) {
  .app-header { padding: 0.6rem 0.75rem; }
  .header-logo { width: 28px; height: 28px; max-width: 28px; max-height: 28px; }
  .header-title { font-size: 0.95rem; }
  .header-top { gap: 0.5rem; margin-bottom: 0.4rem; }
  .app-main { padding: 0.75rem; }
  .nav-btn { font-size: 0.7rem; padding: 0.35rem 0.2rem; }
  .main-nav { gap: 0.15rem; }
  /* Show short labels on mobile */
  .nav-short { display: inline; }
  .nav-full { display: none; }
}