/* ===========================
   Fourier Phase-Magnitude Swap
   Premium Dark Theme
   =========================== */

:root {
    --bg-primary: #0d1117;
    --bg-secondary: #161b22;
    --bg-card: rgba(22, 27, 34, 0.85);
    --bg-glass: rgba(30, 40, 55, 0.6);
    --border-color: rgba(100, 140, 200, 0.15);
    --text-primary: #e6edf3;
    --text-secondary: #8b949e;
    --accent-phase: #58a6ff;
    --accent-mag: #f78166;
    --accent-green: #3fb950;
    --accent-purple: #bc8cff;
    --accent-gradient: linear-gradient(135deg, #58a6ff 0%, #bc8cff 100%);
    --swap-gradient: linear-gradient(135deg, #f78166 0%, #58a6ff 50%, #bc8cff 100%);
    --shadow-glow: 0 0 30px rgba(88, 166, 255, 0.15);
    --radius: 12px;
    --transition: 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    min-height: 100vh;
    overflow-x: hidden;
}

/* Background subtle grid pattern */
body::before {
    content: '';
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    background-image:
        radial-gradient(circle at 25% 25%, rgba(88, 166, 255, 0.03) 0%, transparent 50%),
        radial-gradient(circle at 75% 75%, rgba(188, 140, 255, 0.03) 0%, transparent 50%);
    pointer-events: none;
    z-index: 0;
}

/* ---- HEADER ---- */
header {
    position: relative;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 18px 20px;
    background: var(--bg-secondary);
    border-bottom: 1px solid var(--border-color);
    z-index: 10;
}

header h1 {
    font-size: 1.2rem;
    font-weight: 700;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    text-align: center;
}

.back-btn {
    position: absolute;
    right: 16px;
    top: 50%;
    transform: translateY(-50%);
    background: var(--bg-glass);
    border: 1px solid var(--border-color);
    color: var(--text-secondary);
    padding: 8px 16px;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.85rem;
    font-family: inherit;
    transition: var(--transition);
    backdrop-filter: blur(8px);
}
.back-btn:hover {
    color: var(--text-primary);
    border-color: var(--accent-phase);
    background: rgba(88, 166, 255, 0.1);
}

/* ---- MAIN ---- */
main {
    position: relative;
    z-index: 1;
    max-width: 900px;
    margin: 0 auto;
    padding: 20px 16px 40px;
}

/* ---- IMAGE SELECTORS ---- */
.image-selectors {
    display: flex;
    gap: 30px;
    justify-content: center;
    flex-wrap: wrap;
    margin-bottom: 20px;
    padding: 10px;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    backdrop-filter: blur(8px);
}

.selector-box {
    display: flex;
    align-items: center;
    gap: 12px;
}

.selector-box label {
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--text-secondary);
}

.image-select {
    padding: 10px 16px;
    background: var(--bg-secondary);
    border: 1px solid rgba(88, 166, 255, 0.3);
    border-radius: 8px;
    color: var(--text-primary);
    font-family: inherit;
    font-size: 0.9rem;
    font-weight: 500;
    cursor: pointer;
    outline: none;
    transition: var(--transition);
    min-width: 180px;
    appearance: none;
    background-image: url("data:image/svg+xml;charset=UTF-8,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%2358a6ff' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3e%3cpolyline points='6 9 12 15 18 9'%3e%3c/polyline%3e%3c/svg%3e");
    background-repeat: no-repeat;
    background-position: right 12px center;
    background-size: 16px;
}

.image-select:hover, .image-select:focus {
    border-color: var(--accent-phase);
    box-shadow: 0 0 10px rgba(88, 166, 255, 0.2);
}

.image-select option {
    background: var(--bg-secondary);
    color: var(--text-primary);
    padding: 10px;
}

/* ---- UPLOAD PANEL ---- */
.upload-panel {
    display: none;
    gap: 12px;
    justify-content: center;
    margin-bottom: 20px;
    flex-wrap: wrap;
}
.upload-panel.visible {
    display: flex;
}

.upload-box {
    flex: 1;
    max-width: 220px;
}

.upload-label {
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 14px 18px;
    background: var(--bg-card);
    border: 2px dashed rgba(88, 166, 255, 0.3);
    border-radius: var(--radius);
    color: var(--accent-phase);
    font-size: 0.85rem;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
}
.upload-label:hover {
    border-color: var(--accent-phase);
    background: rgba(88, 166, 255, 0.08);
}
.upload-label input[type="file"] {
    display: none;
}

/* ---- IMAGE GRID ---- */
.image-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 14px;
    margin-bottom: 24px;
}

.grid-cell {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    padding: 10px;
    text-align: center;
    backdrop-filter: blur(8px);
    transition: var(--transition);
}
.grid-cell:hover {
    border-color: rgba(100, 140, 200, 0.3);
}

.cell-label {
    font-size: 0.78rem;
    font-weight: 600;
    color: var(--text-secondary);
    margin-bottom: 8px;
    letter-spacing: 0.02em;
    text-transform: uppercase;
}

.tag {
    display: inline-block;
    font-size: 0.6rem;
    padding: 2px 6px;
    border-radius: 4px;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.04em;
    vertical-align: middle;
}
.phase-tag {
    background: rgba(88, 166, 255, 0.15);
    color: var(--accent-phase);
}
.mag-tag {
    background: rgba(247, 129, 102, 0.15);
    color: var(--accent-mag);
}

.grid-cell canvas {
    width: 100%;
    height: auto;
    border-radius: 6px;
    image-rendering: pixelated;
    background: #000;
}

/* Result cells hidden initially */
.result-cell {
    opacity: 0.3;
    transition: opacity 0.6s ease;
}
.result-cell.revealed {
    opacity: 1;
}

/* ---- SWAP BUTTON ---- */
.swap-section {
    text-align: center;
    margin-bottom: 28px;
}

.swap-btn {
    position: relative;
    display: inline-flex;
    align-items: center;
    gap: 10px;
    padding: 18px 42px;
    font-family: inherit;
    font-size: 1.1rem;
    font-weight: 800;
    color: #fff;
    background: var(--swap-gradient);
    background-size: 200% 200%;
    border: none;
    border-radius: 50px;
    cursor: pointer;
    transition: var(--transition);
    box-shadow: 0 4px 25px rgba(88, 166, 255, 0.3);
    text-transform: uppercase;
    letter-spacing: 0.06em;
}
.swap-btn:disabled {
    opacity: 0.4;
    cursor: not-allowed;
    box-shadow: none;
}
.swap-btn:not(:disabled):hover {
    transform: translateY(-3px) scale(1.03);
    box-shadow: 0 8px 40px rgba(88, 166, 255, 0.4);
    background-position: 100% 100%;
}
.swap-btn:not(:disabled):active {
    transform: translateY(0) scale(0.98);
}
.swap-icon {
    font-size: 1.3rem;
}

.swap-btn.processing {
    animation: pulse-glow 1s ease-in-out infinite;
}
@keyframes pulse-glow {
    0%, 100% { box-shadow: 0 4px 25px rgba(88, 166, 255, 0.3); }
    50% { box-shadow: 0 4px 40px rgba(188, 140, 255, 0.6); }
}

.swap-hint {
    margin-top: 10px;
    font-size: 0.78rem;
    color: var(--text-secondary);
}

/* ---- LESSON PANEL ---- */
.lesson-panel {
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: var(--radius);
    padding: 24px;
    backdrop-filter: blur(8px);
    opacity: 0;
    transform: translateY(15px);
    transition: opacity 0.6s ease, transform 0.6s ease;
    pointer-events: none;
}
.lesson-panel.visible {
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

.lesson-panel h2 {
    font-size: 1rem;
    font-weight: 700;
    margin-bottom: 12px;
    color: var(--accent-phase);
}

.lesson-panel p {
    font-size: 0.88rem;
    line-height: 1.65;
    color: var(--text-secondary);
    margin-bottom: 10px;
}
.lesson-panel p strong {
    color: var(--text-primary);
}
.lesson-panel p em {
    font-style: italic;
}

.lesson-highlight {
    margin-top: 14px;
    padding: 16px 18px;
    background: rgba(63, 185, 80, 0.08);
    border: 1px solid rgba(63, 185, 80, 0.2);
    border-radius: 8px;
    font-size: 0.88rem;
    line-height: 1.65;
    color: var(--text-secondary);
}
.lesson-highlight strong {
    color: var(--accent-green);
}

.phase-color { color: var(--accent-phase); font-weight: 700; }
.mag-color   { color: var(--accent-mag);   font-weight: 700; }

/* ---- RESPONSIVE ---- */
@media (min-width: 768px) {
    header h1 {
        font-size: 1.5rem;
    }
    .image-grid {
        gap: 18px;
    }
    .grid-cell {
        padding: 14px;
    }
}

@media (max-width: 420px) {
    header h1 {
        font-size: 0.95rem;
    }
    .back-btn {
        padding: 6px 10px;
        font-size: 0.75rem;
    }
    .pair-btn {
        min-width: 100px;
        padding: 10px 12px;
        font-size: 0.72rem;
    }
    .swap-btn {
        padding: 14px 28px;
        font-size: 0.95rem;
    }
    .image-grid {
        gap: 8px;
    }
    .grid-cell {
        padding: 6px;
        border-radius: 8px;
    }
    .cell-label {
        font-size: 0.65rem;
    }
    main {
        padding: 12px 8px 30px;
    }
}