/* css/style.css */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap');

:root {
  --primary:       #1e3a5f;
  --accent:        #2563eb;
  --accent-hover:  #1d4ed8;
  --bg:            #f8fafc;
  --surface:       #ffffff;
  --text:          #1e293b;
  --text-muted:    #64748b;
  --border:        #e2e8f0;
  --header-bg:     #1e3a5f;
  --container-max: 1000px;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: 'Inter', Arial, Helvetica, sans-serif;
  color: var(--text);
  background: var(--bg);
  line-height: 1.75;
}

/* ── Header ─────────────────────────────────────────────────── */
.site-header {
  background: var(--header-bg);
  color: #fff;
  width: 100%;
  box-shadow: 0 2px 10px rgba(0,0,0,0.25);
  position: sticky;
  top: 0;
  z-index: 50;
}

.site-header .container {
  max-width: var(--container-max);
  margin: 0 auto;
  padding: 16px 24px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.site-title {
  margin: 0;
  font-size: 1.1rem;
  font-weight: 700;
  color: #fff;
  letter-spacing: 0.03em;
  text-transform: uppercase;
}

/* ── Navigation ──────────────────────────────────────────────── */
nav ul {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  gap: 8px;
  align-items: center;
}

nav a {
  color: rgba(255,255,255,0.82);
  text-decoration: none;
  font-size: 0.875rem;
  font-weight: 500;
  letter-spacing: 0.02em;
  padding: 6px 12px;
  border-radius: 6px;
  transition: background 0.18s, color 0.18s;
}

nav a:hover,
nav a:focus {
  background: rgba(255,255,255,0.12);
  color: #fff;
}

nav a:active {
  color: #93c5fd;
}

/* ── Main content ────────────────────────────────────────────── */
main {
  padding: 2.5rem 1.5rem;
  max-width: var(--container-max);
  margin: 0 auto;
}

/* ── Typography ──────────────────────────────────────────────── */
h1 {
  font-size: 1.75rem;
  font-weight: 700;
  color: var(--primary);
  margin: 0 0 1rem;
  line-height: 1.3;
}

h2 {
  font-size: 1.3rem;
  font-weight: 600;
  color: var(--primary);
  margin: 2rem 0 0.75rem;
  padding-bottom: 0.45rem;
  border-bottom: 2px solid var(--border);
}

h3 {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--primary);
  margin: 1.5rem 0 0.5rem;
}

h4 {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--primary);
  margin: 1.2rem 0 0.4rem;
}

h6 {
  font-size: 0.78rem;
  font-weight: 400;
  color: var(--text-muted);
  font-style: italic;
  margin: 0.5rem 0;
  line-height: 1.5;
}

main p {
  margin-bottom: 1.2rem;
  line-height: 1.8;
  max-width: 900px;
  color: var(--text);
}

ul, ol {
  line-height: 1.9;
  margin-bottom: 1.2rem;
  padding-left: 1.5rem;
}

li {
  margin-bottom: 0.25rem;
}

/* ── Images ──────────────────────────────────────────────────── */
main img {
  max-width: 100%;
  height: auto;
  display: block;
  margin: 0.75rem 0;
  border-radius: 8px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.09);
}

/* ── Links ───────────────────────────────────────────────────── */
a {
  color: var(--accent);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

a:visited {
  color: inherit;
}

/* ── Button ──────────────────────────────────────────────────── */
button {
  background: var(--accent);
  color: #fff;
  border: none;
  padding: 8px 18px;
  border-radius: 6px;
  cursor: pointer;
  font-size: 0.875rem;
  font-weight: 500;
  font-family: 'Inter', Arial, sans-serif;
  letter-spacing: 0.02em;
  transition: background 0.18s;
}

button:hover {
  background: var(--accent-hover);
}

/* ── Utility classes ─────────────────────────────────────────── */
.times { font-family: "Times New Roman", Times, serif; }
.arial { font-family: 'Inter', Arial, Helvetica, sans-serif; }

.centered-text { text-align: center; }
.red-text       { color: #dc2626; }
.black          { color: #1e293b; }

/* ── Row / column layout ─────────────────────────────────────── */
.row {
  display: flex;
  gap: 16px;
  flex-wrap: wrap;
  margin: 1rem 0;
}

.column {
  flex: 1;
  min-width: 200px;
  padding: 8px;
}

/* ── Boxed formula / answer text ─────────────────────────────── */
.boxed-text {
  display: block;
  border: 1px solid var(--border);
  border-left: 4px solid var(--accent);
  background: #eff6ff;
  padding: 14px 18px;
  border-radius: 0 8px 8px 0;
  margin: 12px 0;
  max-width: 100%;
  font-size: 0.95rem;
}

/* ── Right-floated images ────────────────────────────────────── */
.right-img {
  float: right;
  margin-left: 24px;
  margin-bottom: 16px;
  border-radius: 8px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.1);
}

/* ── Toggle link ─────────────────────────────────────────────── */
.toggle-link {
  color: var(--accent);
  text-decoration: underline;
  cursor: pointer;
}

.toggle-link:active {
  color: #7c3aed;
}

/* ── Topics page – topic link cards ─────────────────────────── */
main > nav {
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin-top: 1rem;
  max-width: 540px;
}

main > nav a {
  color: var(--primary);
  text-decoration: none;
  font-weight: 500;
  font-size: 0.95rem;
  padding: 14px 18px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 8px;
  transition: background 0.18s, border-color 0.18s, box-shadow 0.18s;
  display: block;
  box-shadow: 0 1px 3px rgba(0,0,0,0.05);
}

main > nav a:visited {
  color: var(--primary);
}

main > nav a:hover {
  background: #eff6ff;
  border-color: var(--accent);
  box-shadow: 0 2px 8px rgba(37,99,235,0.1);
  text-decoration: none;
}

/* ── Draggable Desmos calculator ─────────────────────────────── */
.draggable-calculator {
  position: absolute;
  top: 100px;
  left: 50px;
  z-index: 1000;
  cursor: move;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--surface);
  box-shadow: 0 8px 28px rgba(0,0,0,0.15);
  overflow: hidden;
}

.draggable-calculator .drag-handle {
  background: var(--header-bg);
  color: #fff;
  padding: 8px 14px;
  cursor: move;
  font-weight: 600;
  font-size: 0.85rem;
  letter-spacing: 0.02em;
}