/* ===== CSS Custom Properties ===== */
:root {
  --color-easy: #16a34a;
  --color-easy-bg: #dcfce7;
  --color-medium: #d97706;
  --color-medium-bg: #fef3c7;
  --color-hard: #dc2626;
  --color-hard-bg: #fee2e2;
  --color-accent: #2563eb;
  --color-accent-bg: #eff6ff;
  --color-surface: #ffffff;
  --color-bg: #f8fafc;
  --color-border: #e2e8f0;
  --color-text: #1e293b;
  --color-text-secondary: #64748b;
  --color-type: #6b7280;
  --color-type-bg: #f3f4f6;
  --color-reveal-bg: #f0fdf4;
  --color-pedagogy-bg: #f8fafc;
  --color-mistakes: #d97706;
  --color-tips: #2563eb;
  --radius: 6px;
  --radius-pill: 999px;
  --sidebar-width: 220px;
  --header-height: 52px;
}

/* ===== Reset ===== */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { font-size: 16px; }
body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  line-height: 1.5;
  min-height: 100vh;
}

/* ===== Utility ===== */
.hidden { display: none !important; }

/* ===== Header ===== */
#site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  height: var(--header-height);
}
#header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 1rem;
  height: 100%;
}
#site-title {
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--color-accent);
  letter-spacing: -0.01em;
}

/* ===== Filter Toggle (mobile only) ===== */
#filter-toggle {
  display: flex;
  align-items: center;
  gap: 0.3rem;
  padding: 0.35rem 0.75rem;
  background: var(--color-accent-bg);
  color: var(--color-accent);
  border: 1px solid var(--color-accent);
  border-radius: var(--radius);
  font-size: 0.875rem;
  font-weight: 500;
  cursor: pointer;
}
#filter-toggle:hover { background: var(--color-accent); color: #fff; }

/* ===== Page Layout ===== */
#page-layout {
  display: flex;
  flex-direction: column;
  min-height: calc(100vh - var(--header-height));
}

/* ===== Filter Panel ===== */
#filter-panel {
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  padding: 1rem;
}
#filter-panel.hidden { display: none !important; }
#filter-panel-inner { display: flex; flex-direction: column; gap: 0; }
.filter-group { margin-bottom: 0.75rem; }
.filter-group label {
  display: block;
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--color-text-secondary);
  margin-bottom: 0.3rem;
}
.filter-group select {
  display: block;
  width: 100%;
  padding: 0.4rem 0.6rem;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  background: var(--color-surface);
  font-size: 0.875rem;
  color: var(--color-text);
  cursor: pointer;
}
.filter-group select:focus { outline: 2px solid var(--color-accent); outline-offset: 1px; }

.clear-btn {
  display: block;
  width: 100%;
  padding: 0.4rem;
  background: transparent;
  color: var(--color-text-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  font-size: 0.8rem;
  cursor: pointer;
  margin-top: 0.25rem;
}
.clear-btn:hover { background: var(--color-bg); color: var(--color-text); }

/* ===== Main Content ===== */
#main-content { display: flex; flex-direction: column; flex: 1; overflow: hidden; }

/* ===== List Panel ===== */
#list-panel {
  flex-shrink: 0;
  background: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  max-height: 40vh;
  overflow-y: auto;
  -webkit-overflow-scrolling: touch;
}
#list-header {
  position: sticky;
  top: 0;
  background: var(--color-surface);
  padding: 0.5rem 1rem;
  border-bottom: 1px solid var(--color-border);
  z-index: 10;
}
#count-label {
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--color-text-secondary);
}

/* ===== Question Cards ===== */
#question-list { padding: 0; }
.question-card {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.4rem;
  padding: 0.65rem 1rem;
  border-bottom: 1px solid var(--color-border);
  cursor: pointer;
  transition: background 0.1s;
}
.question-card:hover { background: var(--color-bg); }
.question-card.active {
  background: var(--color-accent-bg);
  border-left: 3px solid var(--color-accent);
  padding-left: calc(1rem - 3px);
}
.question-id {
  font-family: 'SFMono-Regular', Consolas, monospace;
  font-size: 0.78rem;
  color: var(--color-text-secondary);
  min-width: 8rem;
  flex-shrink: 0;
}
.question-topic {
  font-size: 0.82rem;
  color: var(--color-text);
  flex: 1;
  min-width: 5rem;
}
.question-subtopic {
  font-size: 0.75rem;
  color: var(--color-text-secondary);
  flex: 1;
  min-width: 4rem;
}

/* ===== Chips ===== */
.chip {
  display: inline-block;
  padding: 0.1rem 0.5rem;
  border-radius: var(--radius-pill);
  font-size: 0.7rem;
  font-weight: 600;
  flex-shrink: 0;
  white-space: nowrap;
}
.chip-easy    { background: var(--color-easy-bg);   color: var(--color-easy); }
.chip-medium  { background: var(--color-medium-bg); color: var(--color-medium); }
.chip-hard    { background: var(--color-hard-bg);   color: var(--color-hard); }
.chip-type    { background: var(--color-type-bg);   color: var(--color-type); }

/* ===== No Results ===== */
.no-results {
  padding: 2rem 1rem;
  text-align: center;
  color: var(--color-text-secondary);
  font-size: 0.9rem;
}

/* ===== Question Detail Panel ===== */
#question-detail {
  flex: 1;
  overflow-y: auto;
  -webkit-overflow-scrolling: touch;
  background: var(--color-surface);
  padding: 0;
}
.detail-empty {
  padding: 3rem 1.5rem;
  text-align: center;
  color: var(--color-text-secondary);
  font-size: 0.95rem;
}
#detail-content { padding: 1rem 1rem 2rem; }

/* ===== Back Button (mobile) ===== */
#back-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  padding: 0.35rem 0.75rem;
  background: transparent;
  color: var(--color-accent);
  border: 1px solid var(--color-accent);
  border-radius: var(--radius);
  font-size: 0.85rem;
  cursor: pointer;
  margin-bottom: 1rem;
}
#back-btn:hover { background: var(--color-accent-bg); }

/* ===== Metadata Row ===== */
.metadata-row {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-bottom: 1rem;
  padding-bottom: 0.75rem;
  border-bottom: 1px solid var(--color-border);
  align-items: center;
}
.meta-label {
  font-size: 0.78rem;
  color: var(--color-text-secondary);
  display: flex;
  align-items: center;
  gap: 0.2rem;
}
.meta-sep { color: var(--color-border); font-size: 0.9rem; }

/* ===== Question Statement ===== */
.question-statement {
  font-size: 1rem;
  line-height: 1.7;
  margin-bottom: 1.25rem;
}
.question-statement .katex-display {
  overflow-x: auto;
  max-width: 100%;
  padding: 0.25rem 0;
}

/* ===== Options (MCQ) ===== */
.options-list {
  list-style: none;
  margin-bottom: 1.25rem;
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  overflow: hidden;
}
.options-list li {
  display: flex;
  gap: 0.6rem;
  align-items: baseline;
  padding: 0.6rem 0.75rem;
  border-bottom: 1px solid var(--color-border);
  font-size: 0.95rem;
  line-height: 1.5;
}
.options-list li:last-child { border-bottom: none; }
.options-list li.option-correct { background: var(--color-reveal-bg); }
.option-label {
  font-weight: 700;
  color: var(--color-accent);
  min-width: 1.2rem;
  flex-shrink: 0;
}
.option-text .katex-display {
  overflow-x: auto;
  max-width: 100%;
}

/* ===== Reveal Button ===== */
.reveal-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  padding: 0.55rem 1.1rem;
  background: var(--color-accent);
  color: #fff;
  border: none;
  border-radius: var(--radius);
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  margin-bottom: 1rem;
  transition: background 0.15s;
}
.reveal-btn:hover { background: #1d4ed8; }

/* ===== Answer Section ===== */
.answer-section {
  background: var(--color-reveal-bg);
  border-left: 4px solid var(--color-easy);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 0.75rem 1rem;
  margin-bottom: 1rem;
  font-weight: 700;
  font-size: 1rem;
}
.answer-label {
  display: block;
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--color-easy);
  margin-bottom: 0.25rem;
}
.answer-value { font-size: 1.05rem; }

/* ===== Solution Section ===== */
.solution-section {
  background: var(--color-bg);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  padding: 1rem;
  margin-bottom: 1rem;
  font-size: 0.95rem;
  line-height: 1.7;
}
.solution-heading {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--color-text-secondary);
  margin-bottom: 0.6rem;
  display: block;
}
.solution-section .katex-display {
  overflow-x: auto;
  max-width: 100%;
  padding: 0.25rem 0;
}

/* ===== Pedagogical Sections ===== */
.pedagogy-section {
  border-top: 1px solid var(--color-border);
  padding-top: 0.9rem;
  margin-top: 0.9rem;
}
.pedagogy-heading {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--color-text-secondary);
  margin-bottom: 0.5rem;
}
.pedagogy-body {
  font-size: 0.95rem;
  line-height: 1.7;
}
.pedagogy-body .katex-display {
  overflow-x: auto;
  max-width: 100%;
}
.pedagogy-commonMistakes .pedagogy-heading { color: var(--color-mistakes); }
.pedagogy-examTips .pedagogy-heading       { color: var(--color-tips); }

/* ===== Navigation Controls ===== */
.nav-controls {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  margin-top: 1.25rem;
  padding-top: 0.75rem;
  border-top: 1px solid var(--color-border);
}
.nav-btn {
  padding: 0.4rem 0.9rem;
  background: var(--color-surface);
  color: var(--color-text);
  border: 1px solid var(--color-border);
  border-radius: var(--radius);
  font-size: 0.85rem;
  cursor: pointer;
}
.nav-btn:hover:not(:disabled) { background: var(--color-bg); border-color: var(--color-accent); color: var(--color-accent); }
.nav-btn:disabled { opacity: 0.4; cursor: not-allowed; }
.position-indicator {
  flex: 1;
  text-align: center;
  font-size: 0.8rem;
  color: var(--color-text-secondary);
}

/* ===== Desktop Layout (≥768px) ===== */
@media (min-width: 768px) {
  #filter-toggle { display: none; }
  #filter-panel  { display: block !important; }

  #page-layout {
    flex-direction: row;
    align-items: stretch;
  }

  #filter-panel {
    width: var(--sidebar-width);
    flex-shrink: 0;
    border-bottom: none;
    border-right: 1px solid var(--color-border);
    overflow-y: auto;
    position: sticky;
    top: var(--header-height);
    height: calc(100vh - var(--header-height));
  }

  #main-content {
    flex: 1;
    flex-direction: row;
    overflow: hidden;
  }

  #list-panel {
    width: 340px;
    flex-shrink: 0;
    border-bottom: none;
    border-right: 1px solid var(--color-border);
    max-height: none;
    height: calc(100vh - var(--header-height));
    overflow-y: auto;
  }

  #question-detail {
    flex: 1;
    height: calc(100vh - var(--header-height));
    overflow-y: auto;
  }

  #back-btn { display: none; }

  #detail-content { padding: 1.5rem 2rem 3rem; }
}