:root {
  --bg: #f4f6f9;
  --surface: #ffffff;
  --text: #1a2332;
  --muted: #5c6b7a;
  --accent: #2563eb;
  --accent-soft: #dbeafe;
  --border: #e2e8f0;
  --success: #059669;
  --radius: 14px;
  --shadow: 0 4px 24px rgba(15, 23, 42, 0.06);
  --font: "Segoe UI", system-ui, -apple-system, sans-serif;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.55;
  min-height: 100vh;
}

.app-header {
  background: linear-gradient(135deg, #1e3a8a 0%, #2563eb 55%, #3b82f6 100%);
  color: #fff;
  padding: 1.75rem 1.25rem 1.25rem;
  text-align: center;
  box-shadow: var(--shadow);
}

.app-header h1 {
  margin: 0 0 0.35rem;
  font-size: clamp(1.35rem, 4vw, 1.85rem);
  font-weight: 700;
  letter-spacing: -0.02em;
}

.app-header p {
  margin: 0;
  opacity: 0.9;
  font-size: 0.95rem;
}

.xy-bar {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  justify-content: center;
  align-items: center;
  margin-top: 1rem;
  padding: 0.75rem 1rem;
  background: rgba(255, 255, 255, 0.12);
  border-radius: var(--radius);
  max-width: 420px;
  margin-left: auto;
  margin-right: auto;
}

.xy-bar label {
  font-size: 0.85rem;
  display: flex;
  align-items: center;
  gap: 0.4rem;
}

.xy-bar input {
  width: 3rem;
  padding: 0.4rem 0.5rem;
  border: none;
  border-radius: 8px;
  font-size: 1rem;
  text-align: center;
}

.xy-bar small {
  width: 100%;
  text-align: center;
  opacity: 0.85;
  font-size: 0.75rem;
}

nav.tabs {
  display: flex;
  gap: 0.35rem;
  padding: 0.75rem 1rem;
  max-width: 960px;
  margin: 0 auto;
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
}

nav.tabs button {
  flex: 1;
  min-width: 7rem;
  padding: 0.65rem 1rem;
  border: none;
  background: var(--surface);
  color: var(--muted);
  border-radius: 999px;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.2s, color 0.2s, transform 0.15s;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.04);
}

nav.tabs button:hover {
  color: var(--accent);
}

nav.tabs button.active {
  background: var(--accent);
  color: #fff;
  transform: translateY(-1px);
}

main {
  max-width: 960px;
  margin: 0 auto;
  padding: 0 1rem 3rem;
}

.panel {
  display: none;
  animation: fadeIn 0.35s ease;
}

.panel.active {
  display: block;
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(8px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.card {
  background: var(--surface);
  border-radius: var(--radius);
  padding: 1.25rem 1.35rem;
  margin-bottom: 1rem;
  box-shadow: var(--shadow);
  border: 1px solid var(--border);
}

.card h2 {
  margin: 0 0 0.5rem;
  font-size: 1.1rem;
  color: var(--accent);
}

.card h3 {
  margin: 1rem 0 0.4rem;
  font-size: 0.95rem;
}

.badge {
  display: inline-block;
  padding: 0.2rem 0.55rem;
  background: var(--accent-soft);
  color: var(--accent);
  border-radius: 6px;
  font-size: 0.75rem;
  font-weight: 600;
  margin-bottom: 0.5rem;
}

.data-pills {
  display: flex;
  flex-wrap: wrap;
  gap: 0.35rem;
  margin: 0.75rem 0;
}

.data-pills span {
  background: var(--bg);
  padding: 0.25rem 0.5rem;
  border-radius: 6px;
  font-size: 0.85rem;
  font-variant-numeric: tabular-nums;
}

ol.tasks {
  margin: 0.5rem 0;
  padding-left: 1.2rem;
}

ol.tasks li {
  margin-bottom: 0.35rem;
}

.solution {
  margin-top: 1rem;
  padding-top: 1rem;
  border-top: 1px dashed var(--border);
}

.solution-toggle {
  background: none;
  border: none;
  color: var(--accent);
  font-weight: 600;
  cursor: pointer;
  padding: 0;
  font-size: 0.95rem;
}

.solution-body {
  margin-top: 0.75rem;
  font-size: 0.9rem;
  color: var(--text);
}

.solution-body pre,
.solution-body .step {
  background: #f8fafc;
  padding: 0.65rem 0.85rem;
  border-radius: 8px;
  margin: 0.4rem 0;
  font-size: 0.82rem;
  overflow-x: auto;
  white-space: pre-wrap;
  word-break: break-word;
}

table.freq {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.8rem;
  margin: 0.75rem 0;
}

table.freq th,
table.freq td {
  border: 1px solid var(--border);
  padding: 0.45rem 0.5rem;
  text-align: center;
}

table.freq th {
  background: var(--accent-soft);
  color: var(--accent);
  font-weight: 600;
}

.meaning {
  background: #ecfdf5;
  border-left: 4px solid var(--success);
  padding: 0.65rem 0.85rem;
  border-radius: 0 8px 8px 0;
  margin-top: 0.5rem;
  font-size: 0.88rem;
}

.sample-grid {
  display: grid;
  gap: 0.75rem;
}

.sample-card {
  cursor: pointer;
  transition: border-color 0.2s, box-shadow 0.2s;
}

.sample-card:hover {
  border-color: var(--accent);
}

.sample-card.open {
  border-color: var(--accent);
  box-shadow: 0 6px 28px rgba(37, 99, 235, 0.12);
}

.sample-card .num {
  font-weight: 700;
  color: var(--accent);
  margin-right: 0.35rem;
}

.formula-section {
  margin-bottom: 1.25rem;
}

.formula-section h2 {
  font-size: 1rem;
  margin: 0 0 0.65rem;
  padding-bottom: 0.35rem;
  border-bottom: 2px solid var(--accent-soft);
  color: var(--text);
}

.formula-item {
  display: grid;
  grid-template-columns: minmax(120px, 1fr) 2fr;
  gap: 0.5rem 1rem;
  padding: 0.55rem 0;
  border-bottom: 1px solid var(--border);
  font-size: 0.9rem;
}

.formula-item:last-child {
  border-bottom: none;
}

.formula-item strong {
  color: var(--muted);
  font-weight: 600;
}

.formula-item code {
  font-family: "Cambria Math", Georgia, serif;
  background: var(--bg);
  padding: 0.15rem 0.4rem;
  border-radius: 4px;
}

.search-box {
  width: 100%;
  padding: 0.65rem 1rem;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: 1rem;
  font-size: 0.95rem;
}

.search-box:focus {
  outline: 2px solid var(--accent-soft);
  border-color: var(--accent);
}

.footer-note {
  text-align: center;
  color: var(--muted);
  font-size: 0.8rem;
  padding: 1rem;
}

@media (max-width: 600px) {
  .formula-item {
    grid-template-columns: 1fr;
  }
}