/* ============ Reset & Base ============ */
*, *::before, *::after {
  box-sizing: border-box;
}

html {
  -webkit-text-size-adjust: 100%;
  text-size-adjust: 100%;
}

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji";
  background-color: #f4f6f8;
  color: #1a1a1a;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
}

.container {
  width: 100%;
  max-width: 800px;
  margin: 0 auto;
  padding: 0 16px;
}

/* ============ Header ============ */
.site-header {
  background: linear-gradient(135deg, #2563eb, #1e40af);
  color: #ffffff;
  padding: 24px 0;
  text-align: center;
}

.logo {
  margin: 0;
  font-size: 28px;
  font-weight: 700;
}

.logo span {
  color: #93c5fd;
}

.tagline {
  margin: 4px 0 0;
  font-size: 14px;
  opacity: 0.9;
}

/* ============ Nav ============ */
.calc-nav {
  background: #ffffff;
  border-bottom: 1px solid #e2e8f0;
  position: sticky;
  top: 0;
  z-index: 10;
  -webkit-position: sticky;
}

.calc-nav .container {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  padding-top: 10px;
  padding-bottom: 10px;
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
}

.nav-btn {
  font-family: inherit;
  font-size: 14px;
  padding: 8px 14px;
  border: 1px solid #cbd5e1;
  border-radius: 999px;
  background: #ffffff;
  color: #334155;
  cursor: pointer;
  white-space: nowrap;
  transition: background-color 0.15s ease, color 0.15s ease, border-color 0.15s ease;
  -webkit-appearance: none;
  appearance: none;
}

.nav-btn:hover {
  background: #eff6ff;
  border-color: #93c5fd;
}

.nav-btn.active {
  background: #2563eb;
  color: #ffffff;
  border-color: #2563eb;
}

.nav-btn:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

/* ============ Main / Panels ============ */
main {
  padding: 24px 16px 40px;
}

.calc-panel {
  display: none;
  background: #ffffff;
  border: 1px solid #e2e8f0;
  border-radius: 12px;
  padding: 24px;
  margin: 0 auto;
  max-width: 100%;
  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.05);
}

.calc-panel.active {
  display: block;
  animation: fadeIn 0.2s ease-in;
}

@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

.calc-panel h2 {
  margin-top: 0;
  font-size: 20px;
  color: #1e293b;
}

.hint {
  font-size: 13px;
  color: #64748b;
  margin-top: -8px;
}

.result .hint {
  margin: 12px 0 0;
  padding-top: 12px;
  border-top: 1px solid #e0f2fe;
}

/* ============ Form ============ */
.form-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 16px;
  margin-bottom: 16px;
}

@media (min-width: 480px) {
  .form-grid {
    grid-template-columns: repeat(2, 1fr);
  }
  .form-grid label.full-width {
    grid-column: 1 / -1;
  }
}

label {
  display: flex;
  flex-direction: column;
  font-size: 14px;
  font-weight: 600;
  color: #334155;
  gap: 6px;
}

input,
select {
  font-family: inherit;
  font-size: 16px;
  padding: 10px 12px;
  border: 1px solid #cbd5e1;
  border-radius: 8px;
  background: #ffffff;
  color: #1a1a1a;
  width: 100%;
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
}

select {
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='8' viewBox='0 0 12 8'%3E%3Cpath fill='%23334155' d='M1 1l5 5 5-5'/%3E%3C/svg%3E");
  background-repeat: no-repeat;
  background-position: right 12px center;
  padding-right: 32px;
}

input:focus,
select:focus {
  outline: 2px solid #2563eb;
  outline-offset: 1px;
  border-color: #2563eb;
}

/* Remove number input spinners inconsistency across browsers (keep usable but consistent) */
input[type="number"] {
  -moz-appearance: textfield;
}

/* ============ Buttons ============ */
.calc-btn {
  font-family: inherit;
  font-size: 15px;
  font-weight: 600;
  padding: 12px 24px;
  border: none;
  border-radius: 8px;
  background: #2563eb;
  color: #ffffff;
  cursor: pointer;
  transition: background-color 0.15s ease;
  -webkit-appearance: none;
  appearance: none;
}

.calc-btn:hover {
  background: #1d4ed8;
}

.calc-btn:active {
  background: #1e40af;
}

.calc-btn:focus-visible {
  outline: 2px solid #1e40af;
  outline-offset: 2px;
}

/* ============ Result ============ */
.result {
  margin-top: 16px;
  padding: 16px;
  border-radius: 8px;
  background: #f0f9ff;
  border: 1px solid #bae6fd;
  font-size: 15px;
  color: #0c4a6e;
  min-height: 24px;
}

.result:empty {
  display: none;
}

.result strong {
  display: block;
  font-size: 17px;
  margin-bottom: 4px;
  color: #075985;
}

.result .error {
  color: #b91c1c;
}

.result table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 8px;
}

.result td {
  padding: 4px 0;
  border-bottom: 1px solid #e0f2fe;
}

.result td:last-child {
  text-align: right;
  font-weight: 600;
}

/* ============ Ad Slot ============ */
.ad-slot {
  margin: 24px auto;
  min-height: 1px;
  text-align: center;
}

/* ============ Subtab Navigation (Advanced Math) ============ */
.subtab-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-bottom: 16px;
  border-bottom: 1px solid #e2e8f0;
  padding-bottom: 12px;
}

.subtab-btn {
  font-family: inherit;
  font-size: 13px;
  padding: 6px 12px;
  border: 1px solid #cbd5e1;
  border-radius: 6px;
  background: #f8fafc;
  color: #334155;
  cursor: pointer;
  transition: background-color 0.15s ease, color 0.15s ease, border-color 0.15s ease;
  -webkit-appearance: none;
  appearance: none;
}

.subtab-btn:hover {
  background: #eff6ff;
  border-color: #93c5fd;
}

.subtab-btn.active {
  background: #1e40af;
  color: #ffffff;
  border-color: #1e40af;
}

.subtab-btn:focus-visible {
  outline: 2px solid #2563eb;
  outline-offset: 2px;
}

.subtab-panel {
  display: none;
}

.subtab-panel.active {
  display: block;
  animation: fadeIn 0.2s ease-in;
}

.result .step {
  padding: 4px 0;
}

.result .step-label {
  font-weight: 600;
  color: #075985;
}

.result ol.steps {
  margin: 8px 0 0;
  padding-left: 20px;
}

.result ol.steps li {
  margin-bottom: 6px;
}

.result code,
.result .math-expr {
  font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, "Liberation Mono", monospace;
  background: #e0f2fe;
  padding: 2px 5px;
  border-radius: 4px;
  font-size: 0.95em;
}

.result table.bigo-table {
  font-size: 13px;
}

.result table.bigo-table td:first-child {
  font-weight: 600;
}

.result table.bigo-table td:last-child {
  text-align: left;
  font-weight: normal;
}

/* ============ Footer ============ */
.site-footer {
  background: #1e293b;
  color: #cbd5e1;
  text-align: center;
  padding: 20px 0;
  font-size: 13px;
}

.site-footer a {
  color: #93c5fd;
  text-decoration: none;
}

.site-footer a:hover {
  text-decoration: underline;
}

.site-footer p {
  margin: 4px 0;
}