:root {
  --bg: #ffffff;
  --card: #ffffff;
  --ink: #1f2937;
  --muted: #6b7280;
  --primary: #3b82f6;
  --primary-dark: #2563eb;
  --primary-darker: #1d4ed8;
  --danger: #ef4444;
  --danger-dark: #dc2626;
  --border: #e5e7eb;
  --accent: #dbeafe;
  --shadow: 0 10px 30px rgba(15, 23, 42, 0.08);
  --radius: 12px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, "Noto Sans",
    Arial, "Noto Sans Arabic", "Noto Sans Devanagari", "Noto Sans Thai", sans-serif;
  color: var(--ink);
  background: var(--bg);
  line-height: 1.6;
}

html[dir="rtl"] body { direction: rtl; }

.container {
  width: 100%;
  max-width: 880px;
  margin: 0 auto;
  padding: 0 20px;
}

/* Header */
.site-header {
  background: #ffffff;
  border-bottom: 1px solid var(--border);
  box-shadow: 0 2px 14px rgba(12, 90, 105, 0.06);
  position: sticky;
  top: 0;
  z-index: 10;
}
.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 64px;
}
.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  font-size: 1.15rem;
  color: var(--ink);
  text-decoration: none;
}
.brand-logo {
  width: 40px;
  height: 40px;
  object-fit: contain;
  display: block;
}
.lang-switcher select {
  appearance: none;
  background: var(--accent);
  border: 1px solid var(--border);
  color: var(--ink);
  padding: 8px 30px 8px 12px;
  border-radius: 10px;
  font-size: 0.95rem;
  cursor: pointer;
  background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 12 12'><path fill='%236b7280' d='M2 4l4 4 4-4'/></svg>");
  background-repeat: no-repeat;
  background-position: right 10px center;
}
html[dir="rtl"] .lang-switcher select {
  background-position: left 10px center;
  padding: 8px 12px 8px 30px;
}

/* Hero */
.hero { text-align: center; padding: 40px 0 18px; }
.hero h1, .hero h2 {
  font-size: 2rem;
  margin: 0 0 12px;
  font-weight: 600;
}
.lead { color: var(--muted); max-width: 620px; margin: 0 auto; }

/* Calculator */
.calculator {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 32px 24px;
  margin: 10px 0 36px;
}
.proportion {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 22px;
  flex-wrap: wrap;
}
.fraction {
  display: flex;
  flex-direction: column;
  align-items: stretch;
  gap: 8px;
  width: 130px;
}
.fraction input {
  width: 100%;
  text-align: center;
  font-size: 1.4rem;
  padding: 12px 10px;
  border: 2px solid var(--border);
  border-radius: 10px;
  outline: none;
  transition: border-color .15s, box-shadow .15s;
}
.fraction input:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 4px rgba(59, 130, 246, 0.15);
}
.fraction input.solved {
  background: #e9ecef;
  border-color: #d1d5db;
  color: #374151;
  font-weight: 600;
}
.fraction .bar {
  height: 0;
  border-top: 2px dashed #cbd5e1;
}
.equals { font-size: 2rem; font-weight: 700; color: var(--ink); }

.hint { text-align: center; color: var(--muted); font-size: 0.9rem; margin: 18px 0 0; }

.actions {
  display: flex;
  justify-content: center;
  gap: 12px;
  margin-top: 18px;
}
.btn {
  border: none;
  border-radius: 10px;
  padding: 12px 26px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background .15s, transform .05s;
}
.btn:active { transform: translateY(1px); }
.btn-primary { background: var(--primary); color: #fff; }
.btn-primary:hover { background: var(--primary-dark); }
.btn-secondary { background: var(--danger); color: #fff; }
.btn-secondary:hover { background: var(--danger-dark); }

.result {
  margin-top: 26px;
  text-align: center;
  font-size: 1.25rem;
  font-weight: 500;
  color: var(--ink);
}
.result .var { color: var(--primary-dark); font-weight: 700; }
.error {
  margin-top: 22px;
  text-align: center;
  color: #991b1b;
  background: #fef2f2;
  border: 1px solid #fecaca;
  border-radius: 10px;
  padding: 14px;
}
.work {
  margin-top: 14px;
  background: var(--accent);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 14px 18px;
  font-size: 0.95rem;
  color: var(--muted);
}
.work code { color: var(--ink); font-weight: 600; }
.result .var { display: inline-block; }
.ratio-note {
  display: block;
  margin-top: 6px;
  font-size: 0.95rem;
  color: var(--muted);
  font-weight: 500;
}
.steps-list {
  margin: 8px 0 0;
  padding-left: 20px;
}
html[dir="rtl"] .steps-list { padding-left: 0; padding-right: 20px; }
.steps-list li { margin: 2px 0; }

/* Mode tabs */
.mode-tabs {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  justify-content: center;
  margin-bottom: 20px;
}
.mode-tab {
  border: 1px solid var(--border);
  background: #fff;
  color: var(--primary-dark);
  padding: 8px 16px;
  border-radius: 999px;
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
  transition: background .12s, color .12s, border-color .12s;
}
.mode-tab:hover { background: var(--accent); }
.mode-tab.is-active {
  background: var(--primary);
  border-color: var(--primary);
  color: #fff;
}

/* Relation line */
.relation {
  text-align: center;
  font-size: 1.15rem;
  font-weight: 600;
  color: var(--primary-darker);
  letter-spacing: 0.5px;
  margin: 0 0 16px;
  min-height: 1.2em;
}

/* Direct/inverse (4 inputs) and continued (3 inputs) grids */
.quad, .triple {
  display: grid;
  gap: 12px;
  justify-content: center;
  margin: 0 auto;
  max-width: 420px;
}
.quad { grid-template-columns: repeat(2, 1fr); }
.triple { grid-template-columns: repeat(3, 1fr); }
.num {
  width: 100%;
  text-align: center;
  font-size: 1.3rem;
  padding: 12px 10px;
  border: 2px solid var(--border);
  border-radius: 10px;
  outline: none;
  transition: border-color .15s, box-shadow .15s;
}
.num:focus {
  border-color: var(--primary);
  box-shadow: 0 0 0 4px rgba(59, 130, 246, 0.15);
}
.num.solved {
  background: #e9ecef;
  border-color: #d1d5db;
  color: #374151;
  font-weight: 600;
}

/* Compound */
.compound { display: flex; flex-direction: column; gap: 12px; align-items: stretch; max-width: 520px; margin: 0 auto; }
.base-field { display: flex; flex-direction: column; gap: 6px; }
.base-lbl { font-weight: 600; color: var(--primary-dark); font-size: 0.95rem; }
#factorRows { display: flex; flex-direction: column; gap: 10px; }
.factor-row {
  display: grid;
  grid-template-columns: 1fr auto 1fr auto auto;
  align-items: center;
  gap: 8px;
}
.factor-row .num { font-size: 1.05rem; padding: 10px 8px; }
.f-arrow { color: var(--muted); font-weight: 700; }
.f-type {
  padding: 10px 8px;
  border: 2px solid var(--border);
  border-radius: 10px;
  background: #fff;
  font-size: 0.95rem;
  cursor: pointer;
}
.f-remove {
  border: none;
  background: var(--accent);
  color: var(--danger-dark);
  width: 34px;
  height: 34px;
  border-radius: 8px;
  font-size: 1.2rem;
  line-height: 1;
  cursor: pointer;
}
.f-remove:hover { background: #fee2e2; }
.btn-ghost {
  align-self: flex-start;
  background: var(--accent);
  color: var(--primary-dark);
}
.btn-ghost:hover { background: #cfe0fb; }

/* Options row */
.calc-options {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: center;
  gap: 12px 24px;
  margin-top: 20px;
  color: var(--muted);
  font-size: 0.95rem;
}
.opt-toggle, .opt-precision { display: inline-flex; align-items: center; gap: 8px; }
.opt-precision input {
  width: 64px;
  padding: 6px 8px;
  border: 1px solid var(--border);
  border-radius: 8px;
  text-align: center;
}

/* Verification */
.verify {
  margin-top: 16px;
  text-align: center;
  border-radius: 10px;
  padding: 12px 14px;
  font-size: 0.98rem;
}
.verify.ok { background: #ecfdf5; border: 1px solid #a7f3d0; color: #065f46; }
.verify.bad { background: #fef2f2; border: 1px solid #fecaca; color: #991b1b; }

/* Properties reference */
.props-ref {
  margin-top: 20px;
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 10px 16px;
  background: #fff;
}
.props-ref summary {
  cursor: pointer;
  font-weight: 600;
  color: var(--primary-dark);
}
.props-ref ul { margin: 12px 0 4px; padding-left: 20px; color: var(--ink); font-size: 0.95rem; }
html[dir="rtl"] .props-ref ul { padding-left: 0; padding-right: 20px; }
.props-ref li { margin: 6px 0; }

@media (max-width: 520px) {
  .quad, .triple { grid-template-columns: repeat(2, 1fr); max-width: 320px; }
  .factor-row { grid-template-columns: 1fr auto 1fr; grid-auto-rows: auto; }
  .factor-row .f-type { grid-column: 1 / 3; }
  .factor-row .f-remove { grid-column: 3; justify-self: end; }
  .mode-tab { padding: 7px 12px; font-size: 0.9rem; }
}

/* Presets */
.presets {
  margin-bottom: 40px;
  background: var(--accent);
  border-radius: var(--radius);
  padding: 20px 24px;
}
.presets h2 { margin: 0 0 4px; font-size: 1.15rem; color: var(--primary-darker); }
.muted { color: var(--primary-dark); margin: 0 0 12px; font-size: 0.95rem; }
.preset-grid {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 20px;
}
.preset-link {
  background: none;
  border: none;
  padding: 2px 0;
  cursor: pointer;
  font-size: 1.05rem;
  font-weight: 600;
  color: var(--primary-dark);
  transition: color .12s;
}
.preset-link:hover { color: var(--danger); text-decoration: underline; }
.preset-link:active { transform: translateY(1px); }

/* Content / article */
.content {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 32px 36px;
  margin-bottom: 40px;
}
.content h1 {
  color: var(--primary-darker);
  font-size: 2rem;
  line-height: 1.25;
  font-weight: 700;
  margin: 0.2em 0 0.5em;
}
.content h2 {
  margin-top: 1.6em;
  margin-bottom: 0.4em;
  font-size: 1.4rem;
  color: var(--primary-darker);
}
.content h3 {
  margin-top: 1.3em;
  margin-bottom: 0.3em;
  font-size: 1.12rem;
  color: var(--primary-dark);
}
.content p { margin: 0 0 1em; }
/* Lead / meta description paragraphs at the top of the article */
.content > p:first-of-type,
.content > p:nth-of-type(2) {
  color: var(--muted);
  font-size: 1.08rem;
}
.content ul {
  margin: 0 0 1.2em;
  padding-left: 22px;
}
html[dir="rtl"] .content ul { padding-left: 0; padding-right: 22px; }
.content li { margin: 0 0 0.45em; }

.steps, .tips { padding-left: 22px; }
html[dir="rtl"] .steps, html[dir="rtl"] .tips { padding-left: 0; padding-right: 22px; }
.faq details {
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 12px 16px;
  margin-bottom: 10px;
  background: #fff;
}
.faq summary { cursor: pointer; font-weight: 600; }
.faq p { margin: 10px 0 0; color: var(--muted); }

/* Footer */
.site-footer {
  border-top: 1px solid var(--border);
  background: var(--card);
  padding: 24px 0;
  text-align: center;
  color: var(--muted);
}

@media (max-width: 520px) {
  .proportion { gap: 14px; }
  .fraction { width: 110px; }
  .hero h1, .hero h2 { font-size: 1.6rem; }
  .content { padding: 24px 20px; }
  .content h1 { font-size: 1.65rem; }
}