/* ==========================================================================
   Excel Learning Hub — style.css
   Design tokens + base styles + reusable components.
   No page ever gets its own stylesheet — everything is a class here.
   ========================================================================== */

/* ---- Design tokens ------------------------------------------------- */
:root {
  --primary: #107C41;
  --primary-dark: #0B5A2B;
  --primary-light: #EAF6EF;
  --accent: #2563EB;
  --background: #F8FAFC;
  --surface: #FFFFFF;
  --text: #172033;
  --text-muted: #64748B;
  --border: #E2E8F0;
  --success: #16A34A;
  --warning: #D97706;
  --danger: #DC2626;

  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --shadow-sm: 0 1px 2px rgba(23,32,51,0.06);
  --shadow-md: 0 4px 12px rgba(23,32,51,0.08);

  --font-sans: -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --font-mono: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;

  --space-1: 4px; --space-2: 8px; --space-3: 16px;
  --space-4: 24px; --space-5: 40px; --space-6: 64px;

  --container-width: 1160px;
  --header-height: 64px;
}

/* ---- Reset / base ---------------------------------------------------- */
*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font-sans);
  color: var(--text);
  background: var(--background);
  line-height: 1.6;
  font-size: 16px;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
body > #main-content { flex: 1; width: 100%; }
img, svg { max-width: 100%; height: auto; display: block; }
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }
h1, h2, h3, h4 { line-height: 1.25; margin: 0 0 var(--space-3); font-weight: 700; }
h1 { font-size: 2.25rem; }
h2 { font-size: 1.75rem; }
h3 { font-size: 1.25rem; }
p { margin: 0 0 var(--space-3); }
ul, ol { margin: 0 0 var(--space-3); padding-left: 1.25em; }
:focus-visible { outline: 3px solid var(--accent); outline-offset: 2px; }

.container {
  width: 100%;
  max-width: var(--container-width);
  margin: 0 auto;
  padding: 0 var(--space-3);
}

.skip-link {
  position: absolute;
  left: -9999px;
  top: 0;
  background: var(--primary-dark);
  color: #fff;
  padding: var(--space-2) var(--space-3);
  z-index: 1000;
}
.skip-link:focus { left: var(--space-2); top: var(--space-2); }

.text-muted { color: var(--text-muted); }
.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0,0,0,0); white-space: nowrap; border: 0;
}
.is-hidden { display: none !important; }
.section { padding: var(--space-6) 0; }
.section--muted { background: var(--surface); }
.section-heading { text-align: center; max-width: 640px; margin: 0 auto var(--space-5); }
.section-heading p { color: var(--text-muted); }

/* ---- Buttons ----------------------------------------------------------- */
.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  font-weight: 600;
  border-radius: var(--radius-md);
  padding: 12px 22px;
  border: 2px solid transparent;
  cursor: pointer;
  font-size: 1rem;
  transition: background-color .15s ease, color .15s ease, border-color .15s ease;
}
.button:hover { text-decoration: none; }
.button--primary { background: var(--primary); color: #fff; }
.button--primary:hover { background: var(--primary-dark); }
.button--secondary { background: transparent; color: var(--primary-dark); border-color: var(--primary); }
.button--secondary:hover { background: var(--primary-light); }
.button--sm { padding: 8px 16px; font-size: .9rem; }

/* ---- Header / nav -------------------------------------------------- */
.site-header {
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 100;
}
.site-header__inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: var(--header-height);
  gap: var(--space-4);
}
.site-logo {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-weight: 700;
  color: var(--text);
  flex-shrink: 0;
}
.site-logo:hover { text-decoration: none; }

.nav-links ul {
  display: flex;
  list-style: none;
  margin: 0;
  padding: 0;
  gap: var(--space-4);
}
.nav-links a {
  color: var(--text);
  font-weight: 500;
  padding: var(--space-2) 0;
}
.nav-links a[aria-current="page"] { color: var(--primary-dark); border-bottom: 2px solid var(--primary); }

.site-header__actions { display: flex; align-items: center; gap: var(--space-3); }
.icon-button {
  display: inline-flex; align-items: center; justify-content: center;
  width: 36px; height: 36px; border-radius: var(--radius-sm);
  color: var(--text-muted);
}
.icon-button:hover { background: var(--primary-light); color: var(--primary-dark); }

.menu-toggle {
  display: none;
  flex-direction: column;
  justify-content: center;
  gap: 4px;
  width: 36px; height: 36px;
  background: none; border: none; cursor: pointer;
  padding: 0;
}
.menu-toggle span { display: block; width: 22px; height: 2px; background: var(--text); }

/* ---- Footer ------------------------------------------------------- */
.site-footer { background: var(--text); color: #cbd5e1; padding: var(--space-5) 0 var(--space-3); margin-top: var(--space-6); }
.site-footer__grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr 1fr;
  gap: var(--space-4);
  padding-bottom: var(--space-4);
}
.site-footer h3, .site-footer h4 { color: #fff; }
.site-footer h4 { font-size: 1rem; margin-bottom: var(--space-2); }
.site-footer ul { list-style: none; padding: 0; margin: 0; display: flex; flex-direction: column; gap: var(--space-1); }
.site-footer a { color: #cbd5e1; }
.site-footer a:hover { color: #fff; }
.site-footer__bottom { border-top: 1px solid rgba(255,255,255,.12); padding-top: var(--space-3); font-size: .875rem; color: #94a3b8; }

/* ---- Hero ----------------------------------------------------------- */
.hero { padding: var(--space-6) 0; }
.hero__inner { display: grid; grid-template-columns: 1.1fr 0.9fr; gap: var(--space-5); align-items: center; }
.hero h1 { font-size: 2.75rem; }
.hero p.lead { font-size: 1.125rem; color: var(--text-muted); max-width: 46ch; }
.hero__actions { display: flex; gap: var(--space-3); margin-top: var(--space-4); flex-wrap: wrap; }
.hero__graphic { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-lg); box-shadow: var(--shadow-md); padding: var(--space-3); }

/* ---- Cards --------------------------------------------------------- */
.grid { display: grid; gap: var(--space-4); }
.grid--3 { grid-template-columns: repeat(3, 1fr); }
.grid--4 { grid-template-columns: repeat(4, 1fr); }

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: var(--space-4);
  box-shadow: var(--shadow-sm);
}
.card:hover { box-shadow: var(--shadow-md); }
.card:hover a { text-decoration: none; }
.card h3 { margin-bottom: var(--space-2); }
.card p { color: var(--text-muted); font-size: .95rem; }
.card__meta { display: flex; gap: var(--space-2); align-items: center; margin: var(--space-2) 0; flex-wrap: wrap; font-size: .8rem; }
.card__link { font-weight: 600; }

.path-card { text-align: left; border-top: 4px solid var(--primary); }

/* ---- Badges ---------------------------------------------------------- */
.badge {
  display: inline-block;
  font-size: .75rem;
  font-weight: 700;
  padding: 3px 10px;
  border-radius: 999px;
  text-transform: uppercase;
  letter-spacing: .02em;
}
.badge--beginner { background: var(--primary-light); color: var(--primary-dark); }
.badge--intermediate { background: #FEF3C7; color: #92400E; }
.badge--advanced { background: #FEE2E2; color: #991B1B; }
.badge--time { background: var(--background); color: var(--text-muted); border: 1px solid var(--border); }

/* ---- Topic chips ------------------------------------------------------ */
.chip-grid { display: flex; flex-wrap: wrap; gap: var(--space-2); justify-content: center; }
.chip {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 999px;
  padding: 8px 16px;
  font-size: .9rem;
  color: var(--text);
  font-weight: 500;
}
.chip:hover { border-color: var(--primary); color: var(--primary-dark); text-decoration: none; }

/* ---- Formula box ------------------------------------------------------ */
.formula-box {
  background: var(--primary-light);
  border-left: 4px solid var(--primary-dark);
  border-radius: var(--radius-sm);
  padding: var(--space-3);
  margin: var(--space-3) 0;
  position: relative;
}
.formula-box__code {
  font-family: var(--font-mono);
  font-size: .95rem;
  overflow-x: auto;
  white-space: pre;
  padding-right: 90px;
  display: block;
}
.formula-box__copy {
  position: absolute;
  top: var(--space-2);
  right: var(--space-2);
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  font-size: .8rem;
  padding: 4px 10px;
  cursor: pointer;
  font-weight: 600;
  color: var(--primary-dark);
}
.formula-box__copy:hover { background: var(--primary-light); }

/* ---- Shortcut keys --------------------------------------------------- */
.shortcut { display: flex; align-items: center; gap: var(--space-3); padding: var(--space-2) 0; border-bottom: 1px solid var(--border); }
.shortcut__keys { display: flex; gap: 4px; flex-shrink: 0; }
kbd.key {
  font-family: var(--font-mono);
  background: var(--surface);
  border: 1px solid var(--border);
  border-bottom-width: 2px;
  border-radius: var(--radius-sm);
  padding: 3px 8px;
  font-size: .85rem;
}

/* ---- Search box -------------------------------------------------------- */
.search-box { display: flex; max-width: 480px; margin: 0 auto; gap: var(--space-2); }
.search-box input {
  flex: 1;
  padding: 12px 16px;
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  font-size: 1rem;
}
.search-box input:focus { border-color: var(--primary); }

/* ---- FAQ / accordion --------------------------------------------------- */
.faq-item { border-bottom: 1px solid var(--border); padding: var(--space-3) 0; }
.faq-item summary { font-weight: 600; cursor: pointer; list-style: none; }
.faq-item summary::-webkit-details-marker { display: none; }
.faq-item summary::after { content: '+'; float: right; color: var(--primary); font-size: 1.25rem; }
.faq-item[open] summary::after { content: '−'; }
.faq-item p { margin-top: var(--space-2); color: var(--text-muted); }

/* ---- Callout / CTA bands ------------------------------------------------ */
.callout {
  background: var(--primary-dark);
  color: #fff;
  border-radius: var(--radius-lg);
  padding: var(--space-5);
  text-align: center;
}
.callout h2 { color: #fff; }
.callout p { color: rgba(255,255,255,.85); }

/* ---- Why-learn-here icon list ------------------------------------------ */
.feature-list { display: grid; grid-template-columns: repeat(3, 1fr); gap: var(--space-4); }
.feature-list__item { display: flex; gap: var(--space-3); align-items: flex-start; }
.feature-list__icon {
  flex-shrink: 0; width: 40px; height: 40px; border-radius: var(--radius-md);
  background: var(--primary-light); color: var(--primary-dark);
  display: flex; align-items: center; justify-content: center;
}

/* ---- Lesson / course detail layout ------------------------------------- */
.page-header { padding: var(--space-4) 0 var(--space-5); }
.page-header__badges { display: flex; gap: var(--space-2); align-items: center; margin-bottom: var(--space-2); flex-wrap: wrap; }
.page-header p.lead { color: var(--text-muted); font-size: 1.05rem; max-width: 65ch; }

.lesson-layout { display: grid; grid-template-columns: 1fr 240px; gap: var(--space-5); align-items: start; }
.lesson-main { min-width: 0; }
.lesson-main h2 { margin-top: var(--space-5); }
.lesson-main table { width: 100%; border-collapse: collapse; overflow-x: auto; display: block; }
.lesson-main table th, .lesson-main table td { border: 1px solid var(--border); padding: var(--space-2); text-align: left; }

.lesson-sidebar {
  position: sticky;
  top: calc(var(--header-height) + var(--space-3));
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  padding: var(--space-3);
}
.lesson-sidebar__title { font-size: .8rem; font-weight: 700; text-transform: uppercase; color: var(--text-muted); margin: 0 0 var(--space-2); }
.lesson-sidebar ul { list-style: none; margin: 0; padding: 0; display: flex; flex-direction: column; gap: 6px; }
.lesson-sidebar a { color: var(--text); font-size: .9rem; }
.lesson-sidebar a:hover { color: var(--primary-dark); }

.lesson-box { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-md); padding: var(--space-4); margin: var(--space-4) 0; }
.lesson-box h3 { margin-top: 0; }
.lesson-box--exercise { border-left: 4px solid var(--accent); }
.lesson-box--mistakes { border-left: 4px solid var(--warning); }
.lesson-box--takeaways { border-left: 4px solid var(--success); background: var(--primary-light); }

.interactive-demo { background: var(--background); border: 1px solid var(--border); border-radius: var(--radius-md); padding: var(--space-4); margin: var(--space-4) 0; }
.interactive-demo label { display: block; font-weight: 600; margin-bottom: 4px; font-size: .9rem; }
.interactive-demo input[type="number"], .interactive-demo input[type="text"] {
  padding: 8px 12px; border: 1px solid var(--border); border-radius: var(--radius-sm); width: 100%; max-width: 220px; font-size: 1rem;
}
.interactive-demo__result { margin-top: var(--space-3); font-weight: 700; font-family: var(--font-mono); background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-sm); padding: var(--space-2) var(--space-3); display: inline-block; }
.interactive-demo table { width: 100%; border-collapse: collapse; margin-top: var(--space-2); }
.interactive-demo table th, .interactive-demo table td { border: 1px solid var(--border); padding: 6px 10px; text-align: left; font-size: .9rem; }
.interactive-demo table td.is-match { background: var(--primary-light); }

.lesson-nav-buttons { display: grid; grid-template-columns: 1fr 1fr; gap: var(--space-3); margin: var(--space-5) 0; }
.lesson-nav-buttons a { display: flex; flex-direction: column; gap: 4px; border: 1px solid var(--border); border-radius: var(--radius-md); padding: var(--space-3); background: var(--surface); color: var(--text); }
.lesson-nav-buttons a:hover { border-color: var(--primary); text-decoration: none; }
.lesson-nav-buttons__next { text-align: right; grid-column: 2; }

.related-links { border-top: 1px solid var(--border); padding-top: var(--space-4); margin-top: var(--space-4); }
.related-links ul { list-style: none; padding: 0; margin: 0; display: flex; flex-wrap: wrap; gap: var(--space-2); }
.related-links a { background: var(--primary-light); color: var(--primary-dark); padding: 6px 14px; border-radius: 999px; font-size: .9rem; font-weight: 500; }
.related-links a:hover { text-decoration: none; background: var(--primary); color: #fff; }

/* ---- Search-filterable list items (client-side JS enhancement) -------- */
.search-item-wrap { display: contents; }
.search-item-wrap.is-hidden { display: none !important; }

/* ---- Shortcuts library page -------------------------------------------- */
.shortcut-category { margin-bottom: var(--space-5); }
.shortcut-list { display: flex; flex-direction: column; gap: 2px; }
.shortcut { text-decoration: none; color: var(--text); border-radius: var(--radius-sm); }
a.shortcut:hover { background: var(--primary-light); text-decoration: none; }
div.shortcut:not(a) { cursor: default; }

/* ---- Search results page ------------------------------------------------ */
.search-results-group { margin-bottom: var(--space-5); }
.search-results-group h2 { font-size: 1.1rem; text-transform: uppercase; letter-spacing: .03em; color: var(--text-muted); }
.search-result-item { display: block; padding: var(--space-3); border: 1px solid var(--border); border-radius: var(--radius-md); background: var(--surface); margin-bottom: var(--space-2); }
.search-result-item:hover { border-color: var(--primary); text-decoration: none; }
.search-result-item p { margin: 4px 0 0; color: var(--text-muted); font-size: .9rem; }

/* ---- Projects ------------------------------------------------------- */
.project-section { margin: var(--space-5) 0; }
.project-section h2 { display: flex; align-items: center; gap: var(--space-2); }
.sample-data-table { width: 100%; border-collapse: collapse; margin: var(--space-3) 0; }
.sample-data-table th, .sample-data-table td { border: 1px solid var(--border); padding: 8px 12px; text-align: left; font-size: .9rem; }
.sample-data-table th { background: var(--background); }

/* ---- Quiz ------------------------------------------------------------- */
.quiz-question { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-lg); padding: var(--space-4); margin-bottom: var(--space-4); }
.quiz-question__prompt { font-weight: 600; margin-bottom: var(--space-3); }
.quiz-answer { display: flex; align-items: center; gap: var(--space-2); padding: var(--space-2) var(--space-3); border: 1px solid var(--border); border-radius: var(--radius-sm); margin-bottom: 8px; cursor: pointer; }
.quiz-answer:hover { border-color: var(--primary); }
.quiz-answer input { margin: 0; }
.quiz-answer.is-correct { border-color: var(--success); background: #F0FDF4; }
.quiz-answer.is-incorrect { border-color: var(--danger); background: #FEF2F2; }
.quiz-question__explanation { margin-top: var(--space-3); padding: var(--space-3); background: var(--primary-light); border-radius: var(--radius-sm); font-size: .9rem; display: none; }
.quiz-question__explanation.is-visible { display: block; }
.quiz-results-summary { text-align: center; padding: var(--space-5); background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius-lg); margin-bottom: var(--space-4); }
.quiz-results-summary__score { font-size: 2.5rem; font-weight: 700; color: var(--primary-dark); }

/* ---- Progress tracking (localStorage, no accounts) ---------------------- */
.progress-bar-wrap { margin: var(--space-4) 0; }
.progress-bar-wrap__label { display: flex; justify-content: space-between; font-size: .875rem; font-weight: 600; margin-bottom: 6px; }
.progress-bar { height: 10px; background: var(--border); border-radius: 999px; overflow: hidden; }
.progress-bar__fill { height: 100%; background: var(--primary); border-radius: 999px; transition: width .3s ease; }
.lesson-row-complete-icon { display: none; color: var(--success); flex-shrink: 0; }
.is-lesson-complete .lesson-row-complete-icon { display: inline-flex; }
.mark-complete-button { margin: var(--space-4) 0; }
.mark-complete-button.is-complete { background: var(--success); border-color: var(--success); }

/* ---- Breadcrumbs -------------------------------------------------------- */
.breadcrumbs { padding: var(--space-3) 0; font-size: .875rem; }
.breadcrumbs ol { display: flex; flex-wrap: wrap; list-style: none; margin: 0; padding: 0; gap: 6px; }
.breadcrumbs li:not(:last-child)::after { content: '/'; margin-left: 6px; color: var(--text-muted); }
.breadcrumbs a { color: var(--text-muted); }
.breadcrumbs [aria-current="page"] { color: var(--text); font-weight: 600; }