/* ── Design tokens ─────────────────────────────────────────────── */
:root {
    --bg:           #f8fafc;
    --surface:      #ffffff;
    --surface-alt:  #f1f5f9;
    --border:       #e2e8f0;
    --text-primary: #0f172a;
    --text-secondary:#64748b;
    --accent:       #3b82f6;
    --accent-hover: #2563eb;
    --accent-light: #eff6ff;
    --radius-card:  16px;
    --radius-input: 10px;
    --shadow-card:  0 1px 3px rgba(0,0,0,0.07), 0 4px 16px rgba(0,0,0,0.05);
    --shadow-focus: 0 0 0 3px rgba(59,130,246,0.18);
    --font:         'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
}

/* ── Reset ──────────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

body {
    font-family: var(--font);
    background: var(--bg);
    color: var(--text-primary);
    line-height: 1.6;
    -webkit-font-smoothing: antialiased;
    min-height: 100vh;
}

/* ── Layout ─────────────────────────────────────────────────────── */
.container {
    max-width: 760px;
    margin: 0 auto;
    padding: 32px 20px 80px;
}

/* ── Header row ─────────────────────────────────────────────────── */
.header-row {
    display: flex;
    align-items: flex-start;
    justify-content: space-between;
    gap: 16px;
    margin-bottom: 32px;
    flex-wrap: wrap;
}

.header-actions {
    display: flex;
    align-items: center;
    gap: 10px;
    flex-wrap: wrap;
}

.home-btn {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    padding: 6px 14px;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--text-secondary);
    text-decoration: none;
    transition: border-color 0.15s, color 0.15s;
    white-space: nowrap;
}
.home-btn:hover { border-color: var(--accent); color: var(--accent); }

/* ── Language switcher ───────────────────────────────────────────── */
.lang-switcher { display: flex; gap: 6px; flex-wrap: wrap; }

.lang-btn {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 5px 12px;
    font-size: 0.8rem;
    font-family: inherit;
    font-weight: 500;
    cursor: pointer;
    color: var(--text-secondary);
    transition: all 0.15s;
}
.lang-btn:hover  { border-color: var(--accent); color: var(--accent); }
.lang-btn.active { background: var(--accent); border-color: var(--accent); color: #fff; }

/* ── Page heading ───────────────────────────────────────────────── */
header { margin-bottom: 36px; text-align: left; }

h1 {
    font-size: clamp(1.6rem, 4vw, 2.2rem);
    font-weight: 700;
    letter-spacing: -0.02em;
    line-height: 1.2;
    margin-bottom: 8px;
}

.subtitle {
    font-size: 1rem;
    color: var(--text-secondary);
    font-weight: 400;
}

/* ── Calculator section cards ───────────────────────────────────── */
.calc-section {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-card);
    padding: 28px;
    margin-bottom: 24px;
    box-shadow: var(--shadow-card);
}

.section-header { margin-bottom: 20px; }

.section-title {
    font-size: 1.15rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 6px;
    display: flex;
    align-items: center;
    gap: 8px;
}

.section-badge {
    font-size: 0.7rem;
    font-weight: 600;
    padding: 2px 8px;
    border-radius: 99px;
    background: var(--accent-light);
    color: var(--accent);
    letter-spacing: 0.03em;
    text-transform: uppercase;
}

.section-desc {
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.5;
}

/* ── Input grid ─────────────────────────────────────────────────── */
.input-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
    gap: 14px;
    margin-bottom: 20px;
}

.field-group {
    display: flex;
    flex-direction: column;
    gap: 6px;
}

.field-group label {
    font-size: 0.8rem;
    font-weight: 500;
    color: var(--text-secondary);
    letter-spacing: 0.01em;
}

.field-group input {
    width: 100%;
    padding: 10px 14px;
    background: var(--surface-alt);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-input);
    font-size: 0.95rem;
    font-family: inherit;
    color: var(--text-primary);
    transition: border-color 0.15s, box-shadow 0.15s, background 0.15s;
    outline: none;
    -moz-appearance: textfield;
}
.field-group input::-webkit-outer-spin-button,
.field-group input::-webkit-inner-spin-button { -webkit-appearance: none; }

.field-group input:focus {
    border-color: var(--accent);
    box-shadow: var(--shadow-focus);
    background: var(--surface);
}

.field-group input:not(:placeholder-shown):not(:focus) {
    border-color: #cbd5e1;
    background: var(--surface);
}

/* ── Reset button ───────────────────────────────────────────────── */
.reset-row { display: flex; justify-content: flex-end; }

.btn-reset {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    padding: 8px 16px;
    background: transparent;
    border: 1.5px solid var(--border);
    border-radius: 8px;
    font-size: 0.85rem;
    font-weight: 500;
    font-family: inherit;
    color: var(--text-secondary);
    cursor: pointer;
    transition: all 0.15s;
}
.btn-reset:hover { border-color: var(--accent); color: var(--accent); }

/* ── Knowledge card ─────────────────────────────────────────────── */
.knowledge-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-card);
    padding: 28px;
    margin-bottom: 24px;
    box-shadow: var(--shadow-card);
}

.knowledge-card h2 {
    font-size: 1.1rem;
    font-weight: 600;
    margin-bottom: 20px;
}

.knowledge-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 20px;
    margin-bottom: 24px;
}

.knowledge-block h3 {
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--accent);
    margin-bottom: 8px;
}

.knowledge-block p {
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.65;
}

/* ── Conversion table ───────────────────────────────────────────── */
.conv-table-title {
    font-size: 0.9rem;
    font-weight: 600;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.conv-table {
    width: 100%;
    border-collapse: collapse;
    font-size: 0.85rem;
}
.conv-table th {
    text-align: left;
    padding: 8px 12px;
    background: var(--surface-alt);
    font-weight: 600;
    color: var(--text-secondary);
    font-size: 0.78rem;
    border-bottom: 1px solid var(--border);
}
.conv-table td {
    padding: 8px 12px;
    border-bottom: 1px solid var(--border);
    color: var(--text-primary);
}
.conv-table tr:last-child td { border-bottom: none; }
.conv-table tr:hover td { background: var(--surface-alt); }

/* ── FAQ card ───────────────────────────────────────────────────── */
.faq-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-card);
    padding: 28px;
    margin-bottom: 24px;
    box-shadow: var(--shadow-card);
}

.faq-card h2 {
    font-size: 1.1rem;
    font-weight: 600;
    margin-bottom: 20px;
}

.faq-item { margin-bottom: 18px; }
.faq-item:last-child { margin-bottom: 0; }

.faq-item h3 {
    font-size: 0.9rem;
    font-weight: 600;
    margin-bottom: 6px;
    color: var(--text-primary);
}

.faq-item p {
    font-size: 0.875rem;
    color: var(--text-secondary);
    line-height: 1.65;
}

/* ── Footer ─────────────────────────────────────────────────────── */
footer {
    margin-top: 48px;
    text-align: center;
    color: var(--text-secondary);
    font-size: 0.85rem;
    padding-bottom: 32px;
}
footer a { color: inherit; text-decoration: none; }
footer a:hover { color: var(--accent); }

/* ── Responsive ─────────────────────────────────────────────────── */
@media (max-width: 600px) {
    .container { padding-top: 20px; }
    h1 { font-size: 1.5rem; }
    .calc-section, .knowledge-card, .faq-card { padding: 20px; }
    .input-grid { grid-template-columns: 1fr 1fr; }
    .header-row { flex-direction: column-reverse; align-items: flex-start; }
}

@media (max-width: 380px) {
    .input-grid { grid-template-columns: 1fr; }
}