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

:root {
  --bg:        #0d1117;
  --surface:   #161b22;
  --surface2:  #0d1117;
  --border:    #30363d;
  --border2:   #21262d;
  --accent:    #58a6ff;
  --accent2:   #1f6feb;
  --text:      #c9d1d9;
  --text-hi:   #e6edf3;
  --text-lo:   #8b949e;
  --green:     #3fb950;
  --nav-h:     56px;
}

html { scroll-behavior: smooth; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: Arial, Helvetica, sans-serif;
  font-size: 16px;
  line-height: 1.7;
  min-height: 100vh;
  padding-top: var(--nav-h);
}

/* ── Navbar ───────────────────────────────────────────────── */
nav {
  position: fixed;
  top: 0; left: 0; right: 0;
  height: var(--nav-h);
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  padding: 0 32px;
  z-index: 100;
  gap: 0;
}

.nav-brand {
  font-size: 1rem;
  font-weight: 700;
  color: var(--text-hi);
  letter-spacing: 0.06em;
  text-transform: uppercase;
  text-decoration: none;
  margin-right: auto;
  white-space: nowrap;
}

.nav-brand span {
  color: var(--accent);
}

.nav-links {
  display: flex;
  gap: 2px;
  list-style: none;
  margin-left: auto;
}

.nav-links li {
  position: relative;
}

.nav-links .has-submenu > a::after {
  content: " ▼";
  font-size: 0.65em;
  opacity: 0.7;
}

.submenu {
  display: none;
  position: absolute;
  top: calc(100% + 4px);
  left: 0;
  min-width: 150px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  list-style: none;
  padding: 6px 0;
  z-index: 200;
  box-shadow: 0 8px 24px rgba(0,0,0,0.35);
}

.submenu::before {
  content: "";
  position: absolute;
  top: -8px;
  left: 0;
  right: 0;
  height: 8px;
}

.submenu a {
  display: block;
  padding: 8px 14px;
}

.has-submenu.open .submenu {
  display: block;
}

.has-submenu > a {
  cursor: pointer;
}

.nav-links::-webkit-scrollbar {
  display: none;
}

.nav-links a {
  color: var(--text-lo);
  text-decoration: none;
  font-size: 0.82rem;
  padding: 5px 10px;
  border-radius: 6px;
  transition: color 0.15s, background 0.15s;
  white-space: nowrap;
}

.nav-links a:hover,
.nav-links a.active {
  color: var(--text-hi);
  background: var(--border2);
}

.nav-links a.active {
  color: var(--accent);
}

/* ── Page wrapper ─────────────────────────────────────────── */
.page {
  max-width: 860px;
  margin: 0 auto;
  padding: 48px 24px 64px;
}

/* ── Card ─────────────────────────────────────────────────── */
.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 36px 40px;
  margin-bottom: 24px;
}

/* ── Section label ────────────────────────────────────────── */
.label {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 16px;
}

/* ── Divider ──────────────────────────────────────────────── */
hr {
  border: none;
  border-top: 1px solid var(--border);
  margin: 28px 0;
}

/* ── Hero ─────────────────────────────────────────────────── */
.hero {
  text-align: center;
  padding: 48px 24px 36px;
}

.hero h1 {
  font-size: 1.9rem;
  font-weight: 700;
  color: var(--text-hi);
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

.hero .sub {
  font-size: 0.95rem;
  color: var(--text-lo);
  margin-top: 8px;
}

/* ── Researcher card ──────────────────────────────────────── */
.researcher {
  background: var(--surface2);
  border: 1px solid var(--border2);
  border-radius: 6px;
  padding: 20px 24px;
  margin-bottom: 16px;
}

.researcher h3 {
  font-size: 1.05rem;
  color: var(--text-hi);
  margin-bottom: 12px;
}

.researcher .rl {
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.1em;
  color: var(--accent);
  text-transform: uppercase;
  margin-top: 10px;
  margin-bottom: 3px;
}

.researcher p {
  font-size: 0.92rem;
  color: var(--text-lo);
}

/* ── Note box ─────────────────────────────────────────────── */
.note {
  background: #1c2128;
  border-left: 3px solid var(--accent);
  border-radius: 0 4px 4px 0;
  padding: 14px 20px;
  font-size: 0.91rem;
  color: var(--text-lo);
}

.note strong { color: var(--text); }

/* ── Prose ────────────────────────────────────────────────── */
.prose {
  font-size: 0.95rem;
  color: var(--text);
  text-align: justify;
}

/* ── Code block ───────────────────────────────────────────── */
.code-block {
  background: var(--surface2);
  border: 1px solid var(--border2);
  border-radius: 6px;
  padding: 16px 20px;
  font-family: 'Courier New', monospace;
  font-size: 0.85rem;
  color: var(--green);
  white-space: pre;
  overflow-x: auto;
  line-height: 1.5;
}

/* ── Data table ───────────────────────────────────────────── */
table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.88rem;
  margin-top: 12px;
}

th {
  text-align: left;
  color: var(--accent);
  font-size: 0.72rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  padding: 8px 12px;
  border-bottom: 1px solid var(--border);
}

td {
  padding: 8px 12px;
  border-bottom: 1px solid var(--border2);
  color: var(--text-lo);
}

td:first-child { color: var(--text); font-weight: 600; }

tr:last-child td { border-bottom: none; }

/* ── Green numbers in calculation pages ────────────────────── */
.math-inline, .math-display { color: var(--green); }
td code, td strong { color: var(--green); }
.num-green { color: var(--green); }

/* ── Math display & inline ─────────────────────────────────── */
.math-display {
  overflow-x: auto;
  padding: 0.8rem;
  background: rgba(255,255,255,0.04);
  border-radius: 6px;
  font-family: monospace;
  margin: 0.8rem 0;
}

.math-inline {
  font-family: monospace;
  background: rgba(255,255,255,0.06);
  padding: 0 4px;
  border-radius: 3px;
}

/* ── Pre & code blocks ─────────────────────────────────────── */
pre {
  background: rgba(0,0,0,0.3);
  padding: 1rem;
  border-radius: 6px;
  overflow-x: auto;
}

code {
  font-family: monospace;
  font-size: 0.9rem;
}

/* ── Blockquote ────────────────────────────────────────────── */
blockquote {
  border-left: 3px solid var(--accent);
  padding-left: 1rem;
  margin: 1rem 0;
  color: #cbd5e1;
}

/* ── Table hover ───────────────────────────────────────────── */
tr:hover { background: rgba(255,255,255,0.03); }

/* ── Badge ────────────────────────────────────────────────── */
.badge {
  display: inline-block;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  padding: 3px 10px;
  border-radius: 20px;
  background: #1f6feb22;
  color: var(--accent);
  border: 1px solid #1f6feb55;
}

.badge-green {
  background: #3fb95022;
  color: #3fb950;
  border-color: #3fb95055;
}

.badge-yellow {
  background: #d2992222;
  color: #d29922;
  border-color: #d2992255;
}

.badge-orange {
  background: #e3b34122;
  color: #e3b341;
  border-color: #e3b34155;
}

.badge-red {
  background: #f8514922;
  color: #f85149;
  border-color: #f8514955;
}

/* ── Language switcher ─────────────────────────────────────── */
.lang-switch {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 44px;
  padding: 4px 10px;
  border-radius: 20px;
  background: #1f6feb22;
  color: var(--accent);
  border: 1px solid #1f6feb55;
  font-size: 0.72rem;
  font-weight: 700;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  text-decoration: none;
}

.lang-switch:hover {
  background: #1f6feb44;
}

/* ── Millennium problems toggle ─────────────────────────────── */
.problem-item {
  margin-bottom: 24px;
}

.problem-item h3 {
  font-size: 1rem;
  color: var(--text-hi);
  margin-bottom: 8px;
}

.problem-item p {
  font-size: 0.93rem;
  color: var(--text-lo);
}

.btn-toggle {
  display: inline-block;
  padding: 8px 16px;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 6px;
  font-weight: 700;
  cursor: pointer;
  margin-top: 8px;
  transition: background 0.15s;
}

.btn-toggle:hover {
  background: var(--accent2);
}

.solution {
  margin-top: 16px;
  display: none;
}

.solution h4 {
  font-size: 0.95rem;
  color: var(--text-hi);
  margin: 0 0 4px;
}

.solution .subnote {
  font-size: 0.85rem;
  color: var(--text-lo);
  font-style: italic;
  margin-bottom: 10px;
}

.solution ul {
  margin: 8px 0;
  padding-left: 18px;
}

.solution li {
  margin-bottom: 4px;
}

.solution .note + .note {
  margin-top: 12px;
}

/* ── Contact form (Formspree) ──────────────────────────────── */
.contact-form {
  display: flex;
  flex-direction: column;
  gap: 16px;
}

.form-group {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.contact-form label {
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text);
}

.contact-form input,
.contact-form textarea {
  padding: 12px;
  background: var(--surface2);
  border: 1px solid var(--border2);
  border-radius: 6px;
  color: var(--text);
  font-family: inherit;
  font-size: 0.95rem;
}

.contact-form input:focus,
.contact-form textarea:focus {
  outline: none;
  border-color: var(--accent);
}

.contact-form textarea {
  min-height: 120px;
  resize: vertical;
}

.contact-form button[type="submit"] {
  padding: 12px 20px;
  background: var(--accent);
  border: none;
  border-radius: 6px;
  color: #fff;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.15s;
}

.contact-form button[type="submit"]:hover {
  background: var(--accent2);
}

/* ── Footer ───────────────────────────────────────────────── */
footer {
  text-align: center;
  font-size: 0.75rem;
  color: #484f58;
  padding: 32px 0 16px;
  letter-spacing: 0.04em;
}

@media (max-width: 1300px) {
  .nav-links a { padding: 5px 8px; font-size: 0.80rem; }
}

@media (max-width: 720px) {
  nav { padding: 0 12px; }
  .nav-brand { font-size: 0.85rem; margin-right: 8px; }
  .nav-links a { font-size: 0.72rem; padding: 4px 6px; }
}