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

:root {
  --bg: #f8f9fa;
  --surface: #ffffff;
  --ink: #1a1a2e;
  --muted: #6b7280;
  --accent: #2563eb;
  --accent-hover: #1d4ed8;
  --accent-soft: #eff6ff;
  --success: #059669;
  --danger-subtle: #fef2f2;
  --danger-text: #b91c1c;
  --border: #e5e7eb;
  --radius: 12px;
  --radius-sm: 8px;
  --shadow: 0 1px 3px rgba(0,0,0,0.06);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.08);
}

*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
  margin: 0;
  min-height: 100vh;
  font-family: "Inter", system-ui, -apple-system, sans-serif;
  color: var(--ink);
  background: var(--bg);
  font-size: 15px;
  line-height: 1.6;
}

/* ── App shell ── */
.app {
  max-width: 680px;
  margin: 0 auto;
  padding: 32px 20px 48px;
}

.app-header {
  text-align: center;
  margin-bottom: 28px;
}

.app-header h1 {
  margin: 0;
  font-size: 1.5rem;
  font-weight: 700;
}

.subtitle {
  margin: 6px 0 0;
  color: var(--muted);
  font-size: 0.9rem;
}

/* ── Stepper ── */
.stepper {
  display: flex;
  gap: 4px;
  margin-bottom: 20px;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 4px;
  box-shadow: var(--shadow);
}

.step-btn {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 10px 12px;
  border: none;
  border-radius: var(--radius-sm);
  background: transparent;
  color: var(--muted);
  font: inherit;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}

.step-btn:hover { background: var(--accent-soft); color: var(--accent); }
.step-btn.active { background: var(--accent); color: white; }
.step-btn.completed-step { color: var(--success); }

.step-num {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 22px;
  height: 22px;
  border-radius: 50%;
  font-size: 0.75rem;
  font-weight: 700;
  background: rgba(0,0,0,0.06);
  flex-shrink: 0;
}

.step-btn.active .step-num { background: rgba(255,255,255,0.25); }

/* ── Step panels ── */
.step-panel {
  display: none;
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  box-shadow: var(--shadow);
}

.step-panel.active { display: block; }

.step-header { margin-bottom: 16px; }

.step-header h2 {
  margin: 0;
  font-size: 1.15rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  gap: 8px;
}

.step-description {
  margin: 6px 0 0;
  color: var(--muted);
  font-size: 0.88rem;
}

/* ── Help tooltips ── */
.help-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  border: 1.5px solid var(--border);
  background: var(--bg);
  color: var(--muted);
  font-size: 0.7rem;
  font-weight: 700;
  cursor: help;
  position: relative;
  flex-shrink: 0;
  padding: 0;
  line-height: 1;
  font-family: inherit;
  transition: border-color 0.15s, color 0.15s;
}

.help-btn:hover { border-color: var(--accent); color: var(--accent); }
.help-btn.sm { width: 16px; height: 16px; font-size: 0.6rem; }

.tooltip {
  position: absolute;
  bottom: calc(100% + 8px);
  left: 50%;
  transform: translateX(-50%);
  width: 280px;
  padding: 10px 12px;
  background: var(--ink);
  color: #f0f0f0;
  font-size: 0.8rem;
  font-weight: 400;
  line-height: 1.5;
  border-radius: var(--radius-sm);
  box-shadow: 0 8px 24px rgba(0,0,0,0.2);
  z-index: 100;
  pointer-events: none;
}

.tooltip::after {
  content: "";
  position: absolute;
  top: 100%;
  left: 50%;
  transform: translateX(-50%);
  border: 6px solid transparent;
  border-top-color: var(--ink);
}

/* ── Toggle / options ── */
.option-row {
  margin-bottom: 16px;
}

.toggle-label {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 0.88rem;
  font-weight: 500;
  cursor: pointer;
  color: var(--ink);
}

.toggle-label input[type="checkbox"] {
  width: 16px;
  height: 16px;
  accent-color: var(--accent);
  cursor: pointer;
}

.toggle-label span {
  display: flex;
  align-items: center;
  gap: 6px;
}

/* ── Fields & inputs ── */
select, input[type="number"] {
  width: 100%;
  padding: 9px 12px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  font: inherit;
  font-size: 0.9rem;
  color: var(--ink);
  transition: border-color 0.15s, box-shadow 0.15s;
}

select:focus, input[type="number"]:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px rgba(37,99,235,0.12);
}

/* ── Tables ── */
.table-wrap {
  overflow-x: auto;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  margin-bottom: 12px;
}

table {
  width: 100%;
  border-collapse: collapse;
}

th, td {
  padding: 10px 12px;
  border-bottom: 1px solid var(--border);
  text-align: left;
  font-size: 0.88rem;
}

th {
  background: var(--bg);
  font-weight: 600;
  color: var(--muted);
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.03em;
  white-space: nowrap;
}

td input[type="number"] {
  min-width: 120px;
}

tr.is-background {
  background: var(--accent-soft);
}

.row-label {
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.row-number {
  font-weight: 700;
  font-size: 0.9rem;
}

.row-role {
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--muted);
  font-weight: 600;
}

tr.is-background .row-role { color: var(--accent); }

.cal-output {
  font-weight: 700;
  color: var(--accent);
  white-space: nowrap;
}

.cal-output.muted {
  color: var(--muted);
  font-weight: 400;
}

/* ── Fit stats (step 2) ── */
.fit-stats {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
  margin-bottom: 20px;
}

.stat-card {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  padding: 14px;
  text-align: center;
}

.stat-label {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 4px;
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.03em;
  margin-bottom: 4px;
}

.stat-value.big {
  font-size: 1.3rem;
  font-weight: 700;
}

/* ── Equation box ── */
.equation-box {
  background: var(--accent-soft);
  border: 1px solid #bfdbfe;
  border-radius: var(--radius-sm);
  padding: 14px 16px;
  margin-bottom: 20px;
}

.equation-text {
  margin: 8px 0 0;
  font-family: "SF Mono", "Fira Code", "Consolas", monospace;
  font-size: 0.85rem;
  color: var(--ink);
  word-break: break-all;
}

/* ── Chart ── */
.chart-container { margin-bottom: 16px; }

.chart-container h3 {
  margin: 0 0 10px;
  font-size: 1rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  gap: 6px;
}

.chart-box {
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  padding: 12px;
  height: 340px;
}

/* ── Status ── */
.status-msg {
  margin: 0 0 12px;
  font-size: 0.85rem;
  color: var(--muted);
}

/* ── Cal badge ── */
.cal-badge {
  background: var(--accent-soft);
  border: 1px solid #bfdbfe;
  border-radius: var(--radius-sm);
  padding: 8px 14px;
  font-size: 0.82rem;
  color: var(--ink);
  margin-bottom: 16px;
}

.cal-badge.hidden { display: none; }

/* ── Buttons ── */
.btn-primary {
  padding: 10px 20px;
  border: none;
  border-radius: var(--radius-sm);
  background: var(--accent);
  color: white;
  font: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s;
}

.btn-primary:hover { background: var(--accent-hover); }

.btn-secondary {
  padding: 8px 16px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: var(--surface);
  color: var(--ink);
  font: inherit;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
}

.btn-secondary:hover { background: var(--bg); border-color: var(--muted); }

.btn-danger-subtle {
  color: var(--danger-text);
  border-color: #fecaca;
}

.btn-danger-subtle:hover {
  background: var(--danger-subtle);
  border-color: var(--danger-text);
}

.btn-ghost {
  padding: 10px 20px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  background: transparent;
  color: var(--muted);
  font: inherit;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}

.btn-ghost:hover { background: var(--bg); color: var(--ink); }

.btn-row {
  display: flex;
  gap: 8px;
  margin-bottom: 16px;
}

.step-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 20px;
  padding-top: 16px;
  border-top: 1px solid var(--border);
}

/* ── Footer ── */
.app-footer {
  text-align: center;
  margin-top: 24px;
  color: var(--muted);
  font-size: 0.8rem;
}

.app-footer p { margin: 0; }

/* ── Responsive ── */
@media (max-width: 480px) {
  .fit-stats { grid-template-columns: 1fr; }
  .stepper { flex-direction: column; }
  .step-btn { justify-content: flex-start; }
}