/* ===== CSS Variables ===== */
:root {
    --bg-primary: #0a0e27;
    --bg-secondary: #151932;
    --bg-tertiary: #1e2440;
    --accent-primary: #6366f1;
    --accent-secondary: #8b5cf6;
    --accent-gradient: linear-gradient(135deg, #6366f1 0%, #8b5cf6 100%);
    --text-primary: #e2e8f0;
    --text-secondary: #94a3b8;
    --text-muted: #64748b;
    --border-color: rgba(148, 163, 184, 0.1);
    --success: #10b981;
    --warning: #f59e0b;
    --error: #ef4444;
    --glass-bg: rgba(30, 36, 64, 0.7);
    --glass-border: rgba(148, 163, 184, 0.15);
    --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.3);
    --shadow-md: 0 4px 16px rgba(0, 0, 0, 0.4);
    --shadow-lg: 0 8px 32px rgba(0, 0, 0, 0.5);
    --transition-fast: 0.15s ease;
    --transition-normal: 0.3s ease;
    --transition-slow: 0.5s ease;
}

/* ===== Reset & Base Styles ===== */
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    overflow-x: hidden;
}

body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background:
        radial-gradient(circle at 20% 30%, rgba(99, 102, 241, 0.15) 0%, transparent 50%),
        radial-gradient(circle at 80% 70%, rgba(139, 92, 246, 0.15) 0%, transparent 50%);
    pointer-events: none;
    z-index: -1;
}

/* ===== Loading Screen ===== */
#loading-screen {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: var(--bg-primary);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 9999;
}

.loading-content {
    text-align: center;
}

.spinner {
    width: 60px;
    height: 60px;
    margin: 0 auto 2rem;
    border: 4px solid var(--border-color);
    border-top-color: var(--accent-primary);
    border-radius: 50%;
    animation: spin 1s linear infinite;
}

@keyframes spin {
    to {
        transform: rotate(360deg);
    }
}

#loading-status {
    color: var(--text-secondary);
    margin-top: 1rem;
}

.file-upload-btn {
    display: inline-flex;
    align-items: center;
    gap: 0.75rem;
    padding: 1rem 2rem;
    background: var(--accent-gradient);
    border: none;
    border-radius: 12px;
    color: white;
    font-family: inherit;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: all var(--transition-normal);
    box-shadow: var(--shadow-md);
}

.file-upload-btn:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

.file-upload-btn:active {
    transform: translateY(0);
}

/* ===== Header ===== */
.app-header {
    height: 70px;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border-bottom: 1px solid var(--glass-border);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 2rem;
    position: sticky;
    top: 0;
    z-index: 100;
}

.header-left {
    display: flex;
    align-items: center;
    gap: 1rem;
}

.app-title {
    font-size: 1.5rem;
    font-weight: 800;
    letter-spacing: -0.5px;
    background: linear-gradient(135deg, #FF6B6B 0%, #a855f7 50%, #6366f1 100%);
    background-size: 200% auto;
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    animation: shine 4s linear infinite;
    text-shadow: 0 2px 10px rgba(168, 85, 247, 0.2);
}

@keyframes shine {
    to {
        background-position: 200% center;
    }
}

.header-right {
    display: flex;
    align-items: center;
    gap: 1.5rem;
}

.search-container {
    position: relative;
}

#search-input {
    width: 300px;
    padding: 0.625rem 1rem 0.625rem 2.5rem;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    color: var(--text-primary);
    font-size: 0.875rem;
    transition: all var(--transition-normal);
}

#search-input:focus {
    outline: none;
    border-color: var(--accent-primary);
    box-shadow: 0 0 0 3px rgba(99, 102, 241, 0.1);
}

.search-icon {
    position: absolute;
    left: 0.75rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--text-muted);
    pointer-events: none;
}

.stats-badge {
    padding: 0.5rem 1rem;
    background: var(--bg-tertiary);
    border-radius: 20px;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--text-secondary);
}

#total-cards {
    color: var(--accent-primary);
    font-weight: 600;
}

/* ===== Buttons ===== */
.icon-btn,
.text-btn {
    background: transparent;
    border: none;
    color: var(--text-secondary);
    cursor: pointer;
    padding: 0.5rem;
    border-radius: 8px;
    transition: all var(--transition-fast);
    font-family: inherit;
}

.icon-btn:hover,
.text-btn:hover {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.text-btn {
    font-size: 0.875rem;
    font-weight: 500;
}

.sidebar-toggle-btn {
    display: none;
    flex-direction: column;
    justify-content: space-between;
    width: 24px;
    height: 18px;
    background: transparent;
    border: none;
    cursor: pointer;
    padding: 0;
    z-index: 101;
    transition: all var(--transition-normal);
}

.sidebar-toggle-btn span {
    display: block;
    width: 100%;
    height: 2px;
    background-color: var(--text-primary);
    border-radius: 2px;
    transition: all var(--transition-normal);
}

.sidebar-toggle-btn.active span:nth-child(1) {
    transform: translateY(8px) rotate(45deg);
}

.sidebar-toggle-btn.active span:nth-child(2) {
    opacity: 0;
}

.sidebar-toggle-btn.active span:nth-child(3) {
    transform: translateY(-8px) rotate(-45deg);
}

/* ===== Layout ===== */
.app-content {
    display: flex;
    height: calc(100vh - 70px);
}

/* ===== Sidebar ===== */
.sidebar {
    width: 320px;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border-right: 1px solid var(--glass-border);
    overflow-y: auto;
    transition: transform var(--transition-normal);
}

.sidebar-header {
    padding: 1.5rem;
    border-bottom: 1px solid var(--border-color);
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.sidebar-header h3 {
    font-size: 1rem;
    font-weight: 600;
}

.deck-tree {
    padding: 1rem;
}

.deck-item {
    margin-bottom: 0.25rem;
}

.deck-label {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.625rem 0.75rem;
    border-radius: 8px;
    cursor: pointer;
    transition: all var(--transition-fast);
    font-size: 0.875rem;
    user-select: none;
}

.deck-label:hover {
    background: var(--bg-tertiary);
}

.deck-label.active {
    background: var(--accent-gradient);
    color: white;
}

.deck-toggle {
    width: 16px;
    height: 16px;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: transform var(--transition-fast);
}

.deck-toggle.expanded {
    transform: rotate(90deg);
}

.deck-name {
    flex: 1;
}

.deck-count {
    font-size: 0.75rem;
    color: var(--text-muted);
    background: var(--bg-tertiary);
    padding: 0.125rem 0.5rem;
    border-radius: 10px;
}

.deck-children {
    margin-left: 1.25rem;
    margin-top: 0.25rem;
    display: none;
}

.deck-children.expanded {
    display: block;
}

/* ===== Main Area ===== */
.main-area {
    flex: 1;
    overflow-y: auto;
    padding: 2rem;
}

/* ===== Welcome Screen ===== */
.welcome-screen {
    display: flex;
    align-items: center;
    justify-content: center;
    min-height: 100%;
}

.welcome-content {
    text-align: center;
    max-width: 600px;
}

.welcome-content h2 {
    font-size: 2rem;
    margin-bottom: 1rem;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.welcome-content>p {
    color: var(--text-secondary);
    margin-bottom: 3rem;
}

.welcome-stats {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 2rem;
    margin-bottom: 3rem;
}

.stat-item {
    padding: 2rem;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
}

.stat-value {
    font-size: 3rem;
    font-weight: 700;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.stat-label {
    color: var(--text-secondary);
    margin-top: 0.5rem;
}

.keyboard-shortcuts {
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 1px solid var(--glass-border);
    border-radius: 16px;
    padding: 2rem;
}

.keyboard-shortcuts h4 {
    margin-bottom: 1.5rem;
    font-size: 1.125rem;
}

.shortcut-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 1rem;
}

.shortcut-item {
    display: flex;
    align-items: center;
    gap: 1rem;
}

kbd {
    padding: 0.375rem 0.75rem;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 6px;
    font-family: 'Fira Code', monospace;
    font-size: 0.875rem;
    min-width: 2.5rem;
    text-align: center;
}

.shortcut-item span {
    color: var(--text-secondary);
    font-size: 0.875rem;
}

/* ===== Card Display ===== */
.card-display {
    max-width: 900px;
    margin: 0 auto;
}

.card-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 2rem;
}

.breadcrumb {
    color: var(--text-secondary);
    font-size: 0.875rem;
}

.card-counter {
    font-size: 0.875rem;
    color: var(--text-muted);
}

#current-card-num {
    color: var(--accent-primary);
    font-weight: 600;
}

.card-container {
    perspective: 1000px;
    margin-bottom: 2rem;
}

.flashcard {
    position: relative;
    width: 100%;
    min-height: 400px;
    transition: transform var(--transition-slow);
    transform-style: preserve-3d;
    cursor: pointer;
}

.flashcard.flipped {
    transform: rotateY(180deg);
}

.card-face {
    position: absolute;
    width: 100%;
    min-height: 400px;
    backface-visibility: hidden;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 1px solid var(--glass-border);
    border-radius: 20px;
    padding: 3rem;
    box-shadow: var(--shadow-lg);
    display: flex;
    flex-direction: column;
    justify-content: center;
}

.card-back {
    transform: rotateY(180deg);
}

.card-content {
    font-size: 1.125rem;
    line-height: 1.8;
    flex: 1;
    display: flex;
    align-items: center;
    justify-content: center;
    text-align: center;
}

.card-content img {
    max-width: 100%;
    height: auto;
    border-radius: 12px;
    margin: 1rem 0;
}

.flip-hint {
    text-align: center;
    color: var(--text-muted);
    font-size: 0.875rem;
    margin-top: 2rem;
    opacity: 0.7;
}

.cloze-blank {
    color: var(--accent-primary);
    font-weight: 600;
    padding: 0.125rem 0.5rem;
    background: rgba(99, 102, 241, 0.1);
    border-radius: 4px;
}

/* ===== Card Controls ===== */
.card-controls {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 1rem;
}

.nav-btn,
.flip-btn,
.random-btn {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.875rem 1.5rem;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 1px solid var(--glass-border);
    border-radius: 12px;
    color: var(--text-primary);
    font-family: inherit;
    font-size: 0.875rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition-normal);
}

.nav-btn:hover,
.flip-btn:hover,
.random-btn:hover {
    background: var(--bg-tertiary);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.nav-btn:active,
.flip-btn:active,
.random-btn:active {
    transform: translateY(0);
}

.flip-btn {
    background: var(--accent-gradient);
    border: none;
}

.flip-btn:hover {
    background: linear-gradient(135deg, #7c3aed 0%, #a855f7 100%);
}

.nav-btn:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.nav-btn:disabled:hover {
    transform: none;
    background: var(--glass-bg);
}

/* ===== Search Results ===== */
.search-results {
    max-width: 900px;
    margin: 0 auto;
}

.search-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 2rem;
}

.search-header h3 {
    font-size: 1.5rem;
}

.search-results-list {
    display: grid;
    gap: 1rem;
}

.search-result-item {
    padding: 1.5rem;
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border: 1px solid var(--glass-border);
    border-radius: 12px;
    cursor: pointer;
    transition: all var(--transition-normal);
}

.search-result-item:hover {
    background: var(--bg-tertiary);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.search-result-deck {
    color: var(--text-muted);
    font-size: 0.75rem;
    margin-bottom: 0.5rem;
}

.search-result-content {
    font-size: 0.9375rem;
    line-height: 1.6;
}

.search-highlight {
    background: rgba(99, 102, 241, 0.2);
    color: var(--accent-primary);
    padding: 0.125rem 0.25rem;
    border-radius: 3px;
}

/* ===== Scrollbar ===== */
::-webkit-scrollbar {
    width: 8px;
    height: 8px;
}

::-webkit-scrollbar-track {
    background: var(--bg-secondary);
}

::-webkit-scrollbar-thumb {
    background: var(--bg-tertiary);
    border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
    background: var(--text-muted);
}

/* ===== Footer ===== */
.app-footer {
    background: var(--glass-bg);
    backdrop-filter: blur(20px);
    border-top: 1px solid var(--glass-border);
    padding: 3rem 2rem 2rem;
    margin-top: auto;
}

.footer-content {
    max-width: 1200px;
    margin: 0 auto;
}

.footer-content h3 {
    text-align: center;
    font-size: 1.5rem;
    margin-bottom: 2rem;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.footer-stats {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 1.5rem;
    margin-bottom: 2rem;
}

.footer-stat-item {
    padding: 1.5rem;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    display: flex;
    flex-direction: column;
    gap: 0.75rem;
    transition: all var(--transition-normal);
}

.footer-stat-item:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: var(--accent-primary);
}

.stat-number {
    font-size: 2rem;
    font-weight: 700;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.stat-desc {
    color: var(--text-secondary);
    font-size: 0.875rem;
    line-height: 1.6;
}

.footer-bottom {
    text-align: center;
    padding-top: 2rem;
    border-top: 1px solid var(--border-color);
    color: var(--text-muted);
    font-size: 0.875rem;
}

/* ===== Responsive Design ===== */
@media (max-width: 768px) {
    .app-header {
        padding: 0 1rem;
    }

    .sidebar-toggle-btn {
        display: flex;
        margin-right: 1rem;
    }

    .branding-unit {
        flex: 1;
    }

    .app-title {
        font-size: 1.25rem;
        white-space: nowrap;
        overflow: hidden;
        text-overflow: ellipsis;
        max-width: 180px;
    }

    #search-input {
        width: 140px;
        padding-left: 2rem;
        font-size: 0.75rem;
    }

    .search-icon {
        left: 0.5rem;
        width: 16px;
        height: 16px;
    }

    .sidebar {
        position: fixed;
        left: 0;
        top: 70px;
        width: 100%;
        height: calc(100vh - 70px);
        max-width: 320px;
        z-index: 99;
        transform: translateX(-100%);
        box-shadow: var(--shadow-lg);
    }

    .sidebar.open {
        transform: translateX(0);
    }

    .main-area {
        padding: 1rem;
    }

    .card-face {
        padding: 1.5rem;
        min-height: 280px;
    }

    .card-content {
        font-size: 1.125rem;
    }

    .card-controls {
        flex-wrap: wrap;
        gap: 0.5rem;
    }

    .nav-btn, .flip-btn, .random-btn {
        padding: 0.75rem 1rem;
        flex: 1;
        min-width: 100px;
    }

    .nav-btn span,
    .flip-btn span,
    .random-btn span {
        display: inline;
        font-size: 0.8rem;
    }

    .shortcut-grid {
        grid-template-columns: 1fr;
    }

    .welcome-stats {
        grid-template-columns: 1fr;
        gap: 1rem;
    }

    .footer-stats {
        grid-template-columns: 1fr;
    }
}

@media (max-width: 480px) {
    #search-input {
        width: 150px;
    }

    .stats-badge {
        display: none;
    }

    .card-content {
        font-size: 1rem;
    }
}