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

:root {
  --bg: #ffffff;
  --text: #111111;
  --muted: #888888;
  --border: #e8e8e8;
  --hover: #f4f4f4;
  --radius: 4px;
}

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

/* ── Nav ── */
header {
  border-bottom: 1px solid var(--border);
}

nav {
  max-width: 800px;
  margin: 0 auto;
  padding: 1.25rem 2rem;
}

.logo {
  font-size: 0.95rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  color: var(--text);
}

/* ── Main ── */
main {
  flex: 1;
  max-width: 800px;
  margin: 0 auto;
  width: 100%;
  padding: 0 2rem 5rem;
}


.btn {
  display: inline-block;
  border: 1px solid var(--text);
  color: var(--text);
  text-decoration: none;
  padding: 0.55rem 1.25rem;
  border-radius: var(--radius);
  font-size: 0.875rem;
  font-weight: 500;
  transition: background 0.15s, color 0.15s;
}

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

/* ── Cards ── */
.cards {
  display: flex;
  flex-direction: column;
  gap: 0;
}

.card {
  padding: 2rem 0;
  border-bottom: 1px solid var(--border);
  display: grid;
  grid-template-columns: 160px 1fr;
  gap: 0 2rem;
  align-items: start;
}

.card-label {
  font-size: 0.75rem;
  font-weight: 600;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--muted);
  padding-top: 0.15rem;
}

.card h2 {
  font-size: 1rem;
  font-weight: 600;
  margin-bottom: 0.4rem;
}

.card p {
  font-size: 0.9rem;
  color: var(--muted);
  margin-bottom: 0.75rem;
}

.card ul {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
}

.card ul li a {
  font-size: 0.9rem;
  color: var(--text);
  text-decoration: underline;
  text-underline-offset: 3px;
  text-decoration-color: var(--border);
  transition: text-decoration-color 0.15s;
}

.card ul li a:hover {
  text-decoration-color: var(--text);
}

.card .btn {
  margin-top: 0.25rem;
}

/* ── Footer ── */
footer {
  text-align: left;
  padding: 1.5rem 2rem;
  font-size: 0.8rem;
  color: var(--muted);
  border-top: 1px solid var(--border);
  max-width: 800px;
  margin: 0 auto;
  width: 100%;
}

/* ── Breadcrumb ── */
.breadcrumb {
  font-size: 0.85rem;
  color: var(--muted);
  margin-left: 0.5rem;
}

.breadcrumb::before {
  content: "";
  margin-right: 0.5rem;
}

nav a {
  text-decoration: none;
}

/* ── Content Pages ── */
.content {
  padding: 3rem 0;
}

.content h1 {
  font-size: clamp(1.6rem, 4vw, 2.25rem);
  font-weight: 700;
  letter-spacing: -0.5px;
  margin-bottom: 1rem;
  line-height: 1.2;
}

.content h2 {
  font-size: 1.4rem;
  font-weight: 600;
  margin: 2.5rem 0 1rem;
  line-height: 1.3;
}

.content h3 {
  font-size: 1.1rem;
  font-weight: 600;
  margin: 2rem 0 0.75rem;
  line-height: 1.3;
}

.content h4 {
  font-size: 1rem;
  font-weight: 600;
  margin: 1.5rem 0 0.5rem;
}

.content section {
  margin-bottom: 3rem;
}

.content p {
  margin-bottom: 1rem;
  font-size: 0.95rem;
  line-height: 1.7;
}

.content ul, .content ol {
  margin: 1rem 0 1rem 1.5rem;
  line-height: 1.7;
}

.content li {
  margin-bottom: 0.5rem;
  font-size: 0.95rem;
}

.intro {
  font-size: 1.05rem;
  color: var(--muted);
  margin-bottom: 2.5rem;
  line-height: 1.6;
}

/* ── Math & Code Blocks ── */
.math-block {
  margin: 1.25rem 0;
  padding: 1.25rem;
  background: var(--hover);
  border-radius: var(--radius);
  overflow-x: auto;
}

.code-block {
  margin: 1.5rem 0;
  background: var(--hover);
  border-radius: var(--radius);
  overflow-x: auto;
}

.code-block pre {
  padding: 1.25rem;
  font-family: "SF Mono", Monaco, Consolas, monospace;
  font-size: 0.85rem;
  line-height: 1.6;
}

/* ── Highlights & Notes ── */
.highlight {
  padding: 1rem 1.25rem;
  margin: 1rem 0;
  background: var(--hover);
  border-left: 3px solid var(--text);
  border-radius: var(--radius);
}

.highlight strong {
  display: block;
  margin-bottom: 0.25rem;
}

.note {
  font-size: 0.9rem;
  color: var(--muted);
  margin: 0.75rem 0;
  font-style: italic;
}

/* ── Back Link ── */
.back-link {
  display: inline-block;
  margin-top: 3rem;
  font-size: 0.9rem;
  color: var(--muted);
  text-decoration: none;
  transition: color 0.15s;
}

.back-link:hover {
  color: var(--text);
}

/* ── Disabled Cards ── */
.card.disabled {
  opacity: 0.5;
}

.card.disabled .card-label {
  color: var(--border);
}

/* ── Responsive ── */
@media (max-width: 560px) {
  .card {
    grid-template-columns: 1fr;
    gap: 0.5rem 0;
  }
}