:root {
    --primary: #6366f1;
    --primary-dark: #4f46e5;
    --primary-glow: rgba(99, 102, 241, 0.15);
    --bg: #0f172a;
    --bg-card: #1e293b;
    --bg-card-hover: #253349;
    --bg-input: #334155;
    --text: #f1f5f9;
    --text-muted: #94a3b8;
    --text-dim: #64748b;
    --success: #22c55e;
    --success-bg: rgba(34, 197, 94, 0.1);
    --danger: #ef4444;
    --danger-bg: rgba(239, 68, 68, 0.1);
    --warning: #f59e0b;
    --border: #334155;
    --border-light: #475569;
    --radius: 16px;
    --radius-sm: 10px;
    --shadow: 0 4px 24px rgba(0,0,0,0.2);
    --shadow-lg: 0 8px 40px rgba(0,0,0,0.3);
    --transition: 0.25s cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
    font-family: 'Inter', 'SF Pro Display', -apple-system, system-ui, sans-serif;
    background: var(--bg);
    color: var(--text);
    min-height: 100vh;
    min-height: 100dvh;
    overflow-x: hidden;
    -webkit-font-smoothing: antialiased;
}

/* ========== NAV ========== */
.nav {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.8rem 1.5rem;
    background: var(--bg-card);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav-brand {
    font-size: 1.3rem;
    font-weight: 800;
    background: linear-gradient(135deg, var(--primary), #a78bfa);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    letter-spacing: -0.02em;
}

.nav-links {
    display: flex;
    gap: 0.15rem;
    background: var(--bg);
    border-radius: var(--radius-sm);
    padding: 0.2rem;
}

.nav-links button {
    background: none;
    border: none;
    color: var(--text-muted);
    padding: 0.5rem 0.9rem;
    border-radius: 8px;
    cursor: pointer;
    font-size: 0.82rem;
    font-weight: 500;
    transition: var(--transition);
    white-space: nowrap;
}

.nav-links button:hover { color: var(--text); background: var(--bg-input); }
.nav-links button.active {
    background: var(--primary);
    color: white;
    box-shadow: 0 2px 8px rgba(99, 102, 241, 0.4);
}

/* Mobile hamburger */
.nav-hamburger {
    display: none;
    background: none;
    border: none;
    color: var(--text);
    cursor: pointer;
    padding: 0.5rem;
    border-radius: 8px;
    transition: var(--transition);
}
.nav-hamburger:hover { background: var(--bg-input); }

/* ========== MAIN ========== */
.main {
    max-width: 800px;
    margin: 0 auto;
    padding: 1.5rem;
    padding-bottom: 4rem;
}

.page { display: none; animation: fadeIn 0.3s ease; }
.page.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(8px); } to { opacity: 1; transform: translateY(0); } }

/* ========== CARDS ========== */
.card {
    background: var(--bg-card);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 1.5rem;
    margin-bottom: 1rem;
    box-shadow: var(--shadow);
    transition: var(--transition);
}

.card h2 {
    font-size: 1rem;
    font-weight: 700;
    margin-bottom: 1rem;
    color: var(--text);
    letter-spacing: -0.01em;
}
.card h2 .accent { color: var(--primary); }

.card-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1rem;
    flex-wrap: wrap;
    gap: 0.5rem;
}
.card-header h2 { margin: 0; }
.card-header-actions {
    display: flex;
    gap: 0.4rem;
    align-items: center;
    flex-wrap: wrap;
}

/* ========== BUTTONS ========== */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: 0.4rem;
    padding: 0.55rem 1.1rem;
    border: none;
    border-radius: var(--radius-sm);
    cursor: pointer;
    font-size: 0.85rem;
    font-weight: 600;
    transition: var(--transition);
    white-space: nowrap;
}
.btn:active { transform: scale(0.97); }

.btn-primary {
    background: linear-gradient(135deg, var(--primary), #818cf8);
    color: white;
    box-shadow: 0 2px 12px rgba(99, 102, 241, 0.3);
}
.btn-primary:hover { box-shadow: 0 4px 20px rgba(99, 102, 241, 0.45); transform: translateY(-1px); }
.btn-success { background: var(--success); color: white; }
.btn-success:hover { background: #16a34a; }
.btn-danger { background: var(--danger); color: white; }
.btn-danger:hover { background: #dc2626; }
.btn-ghost {
    background: transparent;
    color: var(--text-muted);
    border: 1px solid var(--border);
}
.btn-ghost:hover { background: var(--bg-input); color: var(--text); }

.btn-icon {
    width: 38px; height: 38px; padding: 0;
    display: inline-flex; align-items: center; justify-content: center;
    border-radius: var(--radius-sm); border: none; cursor: pointer;
    background: var(--bg-input); color: var(--text-muted);
    transition: var(--transition);
}
.btn-icon:hover { background: var(--primary); color: white; transform: scale(1.05); }

.btn-playlist {
    display: inline-flex; align-items: center; gap: 0.4rem;
    padding: 0.4rem 0.9rem; border-radius: var(--radius-sm); border: none; cursor: pointer;
    background: var(--bg-input); color: var(--text-muted);
    font-size: 0.85rem; font-weight: 600; transition: var(--transition);
}
.btn-playlist:hover { background: var(--primary); color: white; }
.btn-playlist.playing { background: var(--primary); color: white; }
.playlist-btns { display: flex; gap: 0.4rem; flex-wrap: wrap; }

/* ========== FORMS ========== */
input, select, textarea {
    width: 100%;
    padding: 0.65rem 0.9rem;
    background: var(--bg-input);
    border: 1.5px solid var(--border);
    border-radius: var(--radius-sm);
    color: var(--text);
    font-size: 0.9rem;
    font-family: inherit;
    outline: none;
    transition: var(--transition);
}
input:focus, select:focus, textarea:focus {
    border-color: var(--primary);
    box-shadow: 0 0 0 3px var(--primary-glow);
}
input::placeholder, textarea::placeholder { color: var(--text-dim); }
textarea { resize: vertical; min-height: 80px; }
select { cursor: pointer; }

.form-group { margin-bottom: 0.9rem; }
.form-group label {
    display: block;
    font-size: 0.8rem;
    font-weight: 600;
    color: var(--text-muted);
    margin-bottom: 0.35rem;
    text-transform: uppercase;
    letter-spacing: 0.04em;
}
.form-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 0.8rem;
}

/* ========== VOCAB LIST ========== */
.vocab-item {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.75rem 1rem;
    background: var(--bg);
    border: 1px solid transparent;
    border-radius: var(--radius-sm);
    margin-bottom: 0.4rem;
    transition: var(--transition);
}
.vocab-item:hover {
    background: var(--bg-card-hover);
    border-color: var(--border);
}

.vocab-text { flex: 1; min-width: 0; }
.vocab-viet {
    font-size: 1.05rem;
    font-weight: 600;
    color: var(--warning);
    line-height: 1.3;
}
.vocab-french {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-top: 0.1rem;
}
.vocab-category {
    font-size: 0.7rem;
    font-weight: 600;
    background: var(--primary-glow);
    color: var(--primary);
    padding: 0.2rem 0.55rem;
    border-radius: 99px;
    margin-left: 0.5rem;
    white-space: nowrap;
    border: 1px solid rgba(99, 102, 241, 0.2);
}
.vocab-actions {
    display: flex;
    gap: 0.25rem;
    margin-left: 0.8rem;
    flex-shrink: 0;
}
.vocab-actions .btn-icon { width: 32px; height: 32px; }

/* ========== FLASHCARD ========== */
.flashcard-container {
    perspective: 1200px;
    margin: 1.5rem auto;
    max-width: 440px;
}

.flashcard {
    width: 100%;
    min-height: 220px;
    position: relative;
    cursor: pointer;
    transform-style: preserve-3d;
    transition: transform 0.5s cubic-bezier(0.4, 0, 0.2, 1);
}
.flashcard.flipped { transform: rotateY(180deg); }

.flashcard-face {
    position: absolute;
    width: 100%;
    min-height: 220px;
    backface-visibility: hidden;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    border-radius: var(--radius);
    padding: 2rem 1.5rem;
    text-align: center;
}
.flashcard-front {
    background: linear-gradient(145deg, #f59e0b, #d97706);
    box-shadow: 0 8px 32px rgba(245, 158, 11, 0.3);
}
.flashcard-back {
    background: linear-gradient(145deg, #6366f1, #7c3aed);
    transform: rotateY(180deg);
    box-shadow: 0 8px 32px rgba(99, 102, 241, 0.3);
}
.flashcard-face .word {
    font-size: 1.8rem;
    font-weight: 700;
    margin-bottom: 0.4rem;
    word-break: break-word;
}
.flashcard-face .hint {
    font-size: 0.85rem;
    opacity: 0.75;
}

.flashcard-controls {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: 0.8rem;
    margin-top: 1.5rem;
}

.flashcard-progress {
    text-align: center;
    color: var(--text-dim);
    font-size: 0.85rem;
    font-weight: 500;
}

/* ========== QUIZ ========== */
.quiz-question {
    text-align: center;
    font-size: 1.6rem;
    font-weight: 700;
    color: var(--warning);
    margin: 1.5rem 0;
    word-break: break-word;
}
.quiz-options {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 0.6rem;
    max-width: 480px;
    margin: 0 auto;
}
.quiz-option {
    padding: 0.9rem 0.8rem;
    background: var(--bg);
    border: 2px solid var(--border);
    border-radius: var(--radius-sm);
    cursor: pointer;
    font-size: 0.95rem;
    font-family: inherit;
    color: var(--text);
    transition: var(--transition);
    text-align: center;
    word-break: break-word;
}
.quiz-option:hover { border-color: var(--primary); background: var(--primary-glow); }
.quiz-option.correct {
    border-color: var(--success);
    background: var(--success-bg);
    animation: pop 0.3s ease;
}
.quiz-option.wrong {
    border-color: var(--danger);
    background: var(--danger-bg);
    animation: shake 0.4s ease;
}
@keyframes pop { 50% { transform: scale(1.03); } }
@keyframes shake { 20%, 60% { transform: translateX(-4px); } 40%, 80% { transform: translateX(4px); } }

/* ========== UPLOAD ========== */
.upload-zone {
    border: 2px dashed var(--border-light);
    border-radius: var(--radius);
    padding: 2.5rem 1.5rem;
    text-align: center;
    cursor: pointer;
    transition: var(--transition);
    background: rgba(51, 65, 85, 0.15);
}
.upload-zone:hover,
.upload-zone.dragover {
    border-color: var(--primary);
    background: var(--primary-glow);
    transform: scale(1.01);
}
.upload-zone input { display: none; }
.upload-zone .icon { font-size: 2.2rem; margin-bottom: 0.6rem; }
.upload-zone p { color: var(--text-muted); font-size: 0.9rem; }
.upload-zone .hint { font-size: 0.78rem; color: var(--text-dim); margin-top: 0.4rem; }

.ocr-preview {
    display: none;
    margin-top: 1.2rem;
}
.ocr-preview.visible { display: block; animation: fadeIn 0.3s ease; }

.ocr-entry {
    display: grid;
    grid-template-columns: 1fr 1fr auto auto;
    gap: 0.4rem;
    align-items: center;
    margin-bottom: 0.4rem;
}
.ocr-entry input { font-size: 0.85rem; padding: 0.5rem 0.7rem; }

.ocr-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 0.5rem;
}

.badge {
    font-size: 0.75rem;
    font-weight: 600;
    background: var(--primary-glow);
    color: var(--primary);
    padding: 0.2rem 0.6rem;
    border-radius: 99px;
    border: 1px solid rgba(99, 102, 241, 0.2);
}

/* ========== STATS ========== */
.stat-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 0.8rem;
    margin-bottom: 1.2rem;
}
.stat-box {
    text-align: center;
    padding: 1.2rem 0.8rem;
    background: var(--bg);
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
}
.stat-box .number {
    font-size: 1.8rem;
    font-weight: 800;
    background: linear-gradient(135deg, var(--primary), #a78bfa);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}
.stat-box .label {
    font-size: 0.75rem;
    font-weight: 500;
    color: var(--text-dim);
    margin-top: 0.25rem;
    text-transform: uppercase;
    letter-spacing: 0.04em;
}

/* ========== VERSION BADGE ========== */
.version-badge {
    position: fixed;
    bottom: 0.5rem;
    right: 0.8rem;
    font-size: 0.65rem;
    font-weight: 500;
    color: var(--text-dim);
    opacity: 0.4;
    z-index: 1000;
}

/* ========== TOAST ========== */
.toast {
    position: fixed;
    bottom: 1.5rem;
    left: 50%;
    transform: translateX(-50%) translateY(120px);
    background: var(--bg-card);
    border: 1px solid var(--primary);
    color: var(--text);
    padding: 0.7rem 1.3rem;
    border-radius: var(--radius-sm);
    font-size: 0.85rem;
    font-weight: 500;
    transition: transform 0.35s cubic-bezier(0.4, 0, 0.2, 1);
    z-index: 9999;
    box-shadow: var(--shadow-lg);
    backdrop-filter: blur(10px);
}
.toast.show { transform: translateX(-50%) translateY(0); }

/* ========== LOADING ========== */
.loading {
    display: inline-block;
    width: 28px;
    height: 28px;
    border: 3px solid var(--border);
    border-top-color: var(--primary);
    border-radius: 50%;
    animation: spin 0.7s linear infinite;
}
@keyframes spin { to { transform: rotate(360deg); } }

/* ========== EMPTY STATE ========== */
.empty-state {
    text-align: center;
    padding: 2.5rem 1.5rem;
    color: var(--text-muted);
}
.empty-state .icon {
    font-size: 2.5rem;
    margin-bottom: 0.8rem;
    filter: grayscale(0.3);
}
.empty-state p { font-size: 0.9rem; }

/* ========== SEARCH BAR ========== */
.search-bar {
    position: relative;
}
.search-bar input { padding-left: 2.2rem; }
.search-bar svg {
    position: absolute;
    left: 0.7rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--text-dim);
    pointer-events: none;
}

/* ========== RESPONSIVE ========== */

/* Tablet */
@media (max-width: 768px) {
    .nav { padding: 0.6rem 1rem; }
    .nav-links { gap: 0.1rem; padding: 0.15rem; }
    .nav-links button { padding: 0.4rem 0.6rem; font-size: 0.75rem; }
    .main { padding: 1rem; }
    .card { padding: 1.2rem; }
    .form-row { gap: 0.6rem; }
    .stat-grid { gap: 0.6rem; }
    .stat-box .number { font-size: 1.4rem; }
}

/* Mobile */
@media (max-width: 560px) {
    .nav-hamburger { display: flex; }
    .nav-links {
        display: none;
        position: fixed;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
        background: var(--bg);
        flex-direction: column;
        align-items: center;
        justify-content: center;
        gap: 0.5rem;
        padding: 2rem;
        z-index: 200;
        border-radius: 0;
    }
    .nav-links.open { display: flex; }
    .nav-links button {
        font-size: 1.1rem;
        padding: 0.9rem 2rem;
        width: 100%;
        max-width: 280px;
        text-align: center;
    }

    .main { padding: 0.8rem; padding-bottom: 3rem; }
    .card { padding: 1rem; border-radius: 14px; }
    .form-row { grid-template-columns: 1fr; }
    .quiz-options { grid-template-columns: 1fr; }
    .stat-grid { grid-template-columns: repeat(3, 1fr); }
    .stat-box { padding: 0.8rem 0.4rem; }
    .stat-box .number { font-size: 1.3rem; }
    .stat-box .label { font-size: 0.65rem; }

    .flashcard-face .word { font-size: 1.4rem; }
    .flashcard-face { min-height: 180px; padding: 1.5rem 1rem; }
    .flashcard { min-height: 180px; }

    .vocab-item { padding: 0.6rem 0.7rem; }
    .vocab-viet { font-size: 0.95rem; }
    .vocab-french { font-size: 0.8rem; }
    .vocab-actions .btn-icon { width: 28px; height: 28px; }
    .vocab-actions .btn-icon svg { width: 14px; height: 14px; }

    .card-header { flex-direction: column; align-items: stretch; }
    .card-header-actions { justify-content: stretch; }
    .card-header-actions select,
    .card-header-actions input { width: 100%; }

    .upload-zone { padding: 2rem 1rem; }
    .ocr-entry {
        grid-template-columns: 1fr 1fr;
        gap: 0.3rem;
    }
    .ocr-entry input:nth-child(3) { grid-column: 1 / -1; }
    .ocr-entry button { grid-column: 1 / -1; justify-self: end; }

    .quiz-question { font-size: 1.3rem; margin: 1rem 0; }
}

/* Very small screens */
@media (max-width: 360px) {
    .nav-brand { font-size: 1.1rem; }
    .stat-grid { grid-template-columns: 1fr 1fr 1fr; gap: 0.4rem; }
    .flashcard-face .word { font-size: 1.2rem; }
}