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

:root {
  --bg: #ffffff;
  --text: #1a1a1a;
  --muted: #666666;
  --border: #e0e0e0;
  --border-dark: #bdbdbd;
  --hover: #f5f5f5;
  --selected: #1a1a1a;
  --correct: #e8f5e9;
  --correct-border: #4caf50;
  --wrong: #ffebee;
  --wrong-border: #e53935;
  --easy: #4caf50;
  --medium: #ff9800;
  --hard: #e53935;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
}

body {
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
}

.app {
  max-width: 1400px;
  margin: 0 auto;
  padding: 0 20px 40px;
}

/* Header */
.header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 20px 0;
  border-bottom: 1px solid var(--border);
}

.header h1 {
  font-size: 1.25rem;
  font-weight: 600;
}

.header-right {
  display: flex;
  align-items: center;
  gap: 12px;
}

.status-text {
  font-size: 0.8rem;
  color: var(--muted);
}

/* Toolbar */
.toolbar {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px;
  padding: 16px 0;
  border-bottom: 1px solid var(--border);
}

.search {
  flex: 1;
  min-width: 160px;
  padding: 8px 12px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-size: 0.875rem;
  background: var(--bg);
}

.select {
  padding: 8px 10px;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-size: 0.875rem;
  background: var(--bg);
  color: var(--text);
}

.difficulty-group {
  display: flex;
  border: 1px solid var(--border);
  border-radius: 4px;
  overflow: hidden;
}

.diff-btn {
  padding: 8px 12px;
  border: none;
  border-right: 1px solid var(--border);
  background: var(--bg);
  font-size: 0.8rem;
  cursor: pointer;
  color: var(--text);
}

.diff-btn:last-child {
  border-right: none;
}

.diff-btn:hover {
  background: var(--hover);
}

.diff-btn.active {
  background: var(--selected);
  color: #fff;
}

.count-text {
  font-size: 0.8rem;
  color: var(--muted);
  margin-left: auto;
}

/* Buttons */
.btn {
  padding: 8px 14px;
  border: 1px solid var(--border-dark);
  border-radius: 4px;
  background: var(--bg);
  font-size: 0.875rem;
  cursor: pointer;
  color: var(--text);
}

.btn:hover {
  background: var(--hover);
}

.btn-sm {
  padding: 4px 10px;
  font-size: 0.8rem;
}

.btn-ghost {
  border-color: transparent;
  color: var(--muted);
}

.btn-ghost:hover {
  color: var(--text);
  background: var(--hover);
}

/* Main layout */
.main {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 24px;
  padding-top: 20px;
  align-items: start;
}

@media (max-width: 900px) {
  .main {
    grid-template-columns: 1fr;
  }
}

/* Question grid */
.grid-section {
  min-width: 0;
}

.grid-nav {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 12px;
  font-size: 0.8rem;
  color: var(--muted);
}

.question-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(44px, 1fr));
  gap: 6px;
}

.q-box {
  aspect-ratio: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px solid var(--border);
  border-radius: 4px;
  font-size: 0.8rem;
  font-weight: 500;
  cursor: pointer;
  background: var(--bg);
  color: var(--text);
  position: relative;
}

.q-box:hover {
  background: var(--hover);
  border-color: var(--border-dark);
}

.q-box.active {
  background: var(--selected);
  color: #fff;
  border-color: var(--selected);
}

.q-box.answered-correct {
  border-color: var(--correct-border);
  background: var(--correct);
  color: #1b5e20;
  font-weight: 600;
}

.q-box.answered-wrong {
  border-color: var(--wrong-border);
  background: var(--wrong);
  color: #b71c1c;
  font-weight: 600;
}

.q-box.active.answered-correct {
  background: var(--correct-border);
  border-color: var(--correct-border);
  color: #fff;
}

.q-box.active.answered-wrong {
  background: var(--wrong-border);
  border-color: var(--wrong-border);
  color: #fff;
}

.q-box.diff-E { border-left: 3px solid var(--easy); }
.q-box.diff-M { border-left: 3px solid var(--medium); }
.q-box.diff-H { border-left: 3px solid var(--hard); }

.q-box.active.diff-E,
.q-box.active.diff-M,
.q-box.active.diff-H {
  border-left: 3px solid #fff;
}

.empty-grid {
  grid-column: 1 / -1;
  padding: 40px 0;
  text-align: center;
  color: var(--muted);
  font-size: 0.875rem;
}

.loading-grid {
  grid-column: 1 / -1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 12px;
  padding: 48px 20px;
  text-align: center;
}

.spinner {
  width: 28px;
  height: 28px;
  border: 2px solid var(--border);
  border-top-color: var(--text);
  border-radius: 50%;
  animation: spin 0.7s linear infinite;
}

.loading-label {
  font-size: 0.875rem;
  color: var(--text);
}

.loading-note {
  max-width: 320px;
  font-size: 0.8rem;
  font-style: italic;
  color: var(--muted);
  line-height: 1.5;
}

@keyframes spin {
  to {
    transform: rotate(360deg);
  }
}

/* Viewer */
.viewer {
  border: 1px solid var(--border);
  border-radius: 4px;
  min-height: 400px;
  position: sticky;
  top: 20px;
}

.viewer-empty {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 400px;
  color: var(--muted);
  font-size: 0.875rem;
}

.viewer-body {
  padding: 20px;
}

.viewer-top {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 16px;
  padding-bottom: 12px;
  border-bottom: 1px solid var(--border);
}

.viewer-number {
  font-weight: 600;
  font-size: 0.9rem;
}

.viewer-meta {
  flex: 1;
  min-width: 0;
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.viewer-tags {
  font-size: 0.75rem;
  color: var(--muted);
}

.viewer-ids {
  font-size: 0.72rem;
  color: var(--muted);
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  word-break: break-all;
}

.blank-line {
  display: inline-block;
  min-width: 5rem;
  border-bottom: 1px solid var(--text);
  line-height: 1.2;
  vertical-align: baseline;
}

.viewer-stem math,
.viewer-stimulus math,
.choice-content math,
.viewer-rationale math {
  font-size: 1.05em;
}

.viewer-timer {
  font-size: 0.8rem;
  font-variant-numeric: tabular-nums;
  color: var(--muted);
  min-width: 48px;
  text-align: right;
}

.attempt-log {
  margin-bottom: 14px;
  padding: 10px 12px;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: #fafafa;
  font-size: 0.8rem;
  color: var(--muted);
}

.attempt-log h4 {
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--text);
  margin-bottom: 6px;
  font-weight: 600;
}

.attempt-entry {
  padding: 4px 0;
  border-top: 1px solid var(--border);
}

.attempt-entry:first-of-type {
  border-top: none;
}

.attempt-entry.correct {
  color: #2e7d32;
}

.attempt-entry.wrong {
  color: #c62828;
}

.viewer-stimulus {
  padding: 12px;
  margin-bottom: 16px;
  border-left: 2px solid var(--border-dark);
  font-size: 0.9rem;
  line-height: 1.6;
}

.viewer-stem {
  font-size: 0.95rem;
  line-height: 1.6;
  margin-bottom: 20px;
}

.desmos-panel {
  border: 1px solid var(--border);
  border-radius: 4px;
  position: fixed;
  top: 96px;
  right: 24px;
  width: min(560px, calc(100vw - 32px));
  overflow: hidden;
  background: #fff;
  z-index: 999;
  box-shadow: 0 8px 28px rgba(0, 0, 0, 0.18);
}

.desmos-drag-handle {
  cursor: move;
  user-select: none;
  padding: 8px 10px;
  border-bottom: 1px solid var(--border);
  background: #fafafa;
  font-size: 0.78rem;
  color: var(--muted);
  font-weight: 600;
  letter-spacing: 0.02em;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
}

.desmos-window-actions {
  display: flex;
  align-items: center;
  gap: 6px;
}

.desmos-window-btn {
  width: 28px;
  height: 24px;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: #fff;
  color: var(--text);
  font-size: 14px;
  line-height: 1;
  cursor: pointer;
}

.desmos-window-btn:hover {
  background: var(--hover);
}

.desmos-calculator {
  width: 100%;
  height: 280px;
}

.desmos-panel.expanded .desmos-calculator {
  height: 520px;
}

.viewer-stem p,
.viewer-stimulus p,
.viewer-rationale p {
  margin-bottom: 0.5em;
}

/* Choices */
.choice-list {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.choice-btn {
  display: flex;
  align-items: flex-start;
  gap: 12px;
  width: 100%;
  padding: 12px;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: var(--bg);
  text-align: left;
  cursor: pointer;
  font-size: 0.9rem;
  line-height: 1.5;
  color: var(--text);
}

.choice-btn:hover:not(:disabled) {
  background: var(--hover);
}

.choice-btn.selected {
  border-color: var(--text);
  background: var(--hover);
}

.choice-btn:disabled {
  cursor: default;
}

.choice-btn.correct {
  background: var(--correct);
  border-color: var(--correct-border);
}

.choice-btn.wrong {
  background: var(--wrong);
  border-color: var(--wrong-border);
}

.choice-letter {
  flex-shrink: 0;
  width: 24px;
  height: 24px;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 1px solid var(--border-dark);
  border-radius: 4px;
  font-weight: 600;
  font-size: 0.8rem;
}

.choice-btn.correct .choice-letter {
  background: var(--correct-border);
  border-color: var(--correct-border);
  color: #fff;
}

.choice-btn.wrong .choice-letter {
  background: var(--wrong-border);
  border-color: var(--wrong-border);
  color: #fff;
}

.choice-content {
  flex: 1;
}

/* SPR */
.spr-row {
  display: flex;
  gap: 8px;
}

.check-row {
  margin-top: 12px;
}

.spr-input {
  flex: 1;
  padding: 10px 12px;
  border: 1px solid var(--border-dark);
  border-radius: 4px;
  font-size: 0.95rem;
}

.spr-input.correct {
  border-color: var(--correct-border);
  background: var(--correct);
}

.spr-input.wrong {
  border-color: var(--wrong-border);
  background: var(--wrong);
}

.spr-input:disabled {
  opacity: 1;
}

/* Result banner */
.result-banner {
  margin-top: 16px;
  padding: 10px 12px;
  border-radius: 4px;
  font-size: 0.875rem;
  font-weight: 500;
}

.result-banner.correct {
  background: var(--correct);
  border: 1px solid var(--correct-border);
  color: #2e7d32;
}

.result-banner.wrong {
  background: var(--wrong);
  border: 1px solid var(--wrong-border);
  color: #c62828;
}

.viewer-rationale {
  margin-top: 16px;
  padding-top: 16px;
  border-top: 1px solid var(--border);
  font-size: 0.875rem;
  line-height: 1.6;
  color: var(--muted);
}

.viewer-rationale h4 {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text);
  margin-bottom: 8px;
  font-weight: 600;
}

.hidden {
  display: none !important;
}

.loading {
  opacity: 0.5;
  pointer-events: none;
}

.toast {
  position: fixed;
  bottom: 20px;
  right: 20px;
  padding: 10px 16px;
  background: var(--text);
  color: #fff;
  border-radius: 4px;
  font-size: 0.875rem;
}

/* MathML / tables in content */
.viewer-stem table,
.viewer-stimulus table,
.choice-content table {
  border-collapse: collapse;
  margin: 8px 0;
}

.viewer-stimulus .sr-only,
.viewer-stem .sr-only,
.choice-content .sr-only,
.viewer-rationale .sr-only {
  display: none !important;
}

.viewer-stimulus figure {
  max-width: 100%;
  overflow-x: auto;
}

.viewer-stimulus svg {
  max-width: 100%;
  height: auto;
}

.viewer-stem td,
.viewer-stem th,
.viewer-stimulus td,
.viewer-stimulus th,
.choice-content td,
.choice-content th {
  border: 1px solid var(--border);
  padding: 4px 8px;
}