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

:root {
    /* Facebook-inspired Color Palette */
    --color-primary: #1877f2; /* Facebook blue */
    --color-primary-hover: #166fe5;
    --color-secondary: #e4e6eb;
    --color-success: #42b72a;
    --color-warning: #f7b928;
    --color-error: #fa3e3e;
    --color-success-hover: #36a420;
    --color-warning-hover: #e6a60d;
    --color-error-hover: #e02d2d;
    
    /* Light theme */
    --color-background: #f0f2f5;
    --color-surface: #ffffff;
    --color-surface-hover: #f2f2f2;
    --color-border: #dddfe2;
    --color-text: #050505;
    --color-text-muted: #65676b;
    --color-text-inverse: #ffffff;
    --color-header: #ffffff;
    --color-card: #ffffff;
    --color-card-hover: #f8f9fa;
    --color-shadow: rgba(0, 0, 0, 0.1);

    /* Dark theme */
    --color-background-dark: #18191a;
    --color-surface-dark: #242526;
    --color-surface-hover-dark: #3a3b3c;
    --color-border-dark: #3e4042;
    --color-text-dark: #e4e6eb;
    --color-text-muted-dark: #b0b3b8;
    --color-header-dark: #242526;
    --color-card-dark: #242526;
    --color-card-hover-dark: #3a3b3c;

    /* Typography - Facebook style */
    --font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
    --font-size-xs: 0.75rem;
    --font-size-sm: 0.875rem;
    --font-size-base: 0.9375rem; /* Slightly smaller base font */
    --font-size-md: 1.0625rem;
    --font-size-lg: 1.25rem;
    --font-size-xl: 1.5rem;
    --font-size-2xl: 1.75rem;
    --font-size-3xl: 2rem;
    --font-weight-normal: 400;
    --font-weight-medium: 500;
    --font-weight-bold: 600;

    /* Spacing */
    --space-1: 0.25rem;
    --space-2: 0.5rem;
    --space-3: 0.75rem;
    --space-4: 1rem;
    --space-5: 1.25rem;
    --space-6: 1.5rem;
    --space-8: 2rem;
    --space-10: 2.5rem;
    --space-12: 3rem;
    --space-16: 4rem;

    /* Borders */
    --border-radius: 8px;
    --border-radius-sm: 6px;
    --border-radius-lg: 12px;
    --border-radius-circle: 50%;
    --border-width: 1px;

    /* Shadows */
    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
    --shadow: 0 1px 2px rgba(0, 0, 0, 0.1);
    --shadow-md: 0 2px 4px rgba(0, 0, 0, 0.1);
    --shadow-lg: 0 8px 16px rgba(0, 0, 0, 0.1);
    --shadow-card: 0 1px 2px rgba(0, 0, 0, 0.05);

    /* Transitions */
    --transition: all 0.15s ease-in-out;
}

/* Dark mode */
@media (prefers-color-scheme: dark) {
    :root {
        --color-background: var(--color-background-dark);
        --color-surface: var(--color-surface-dark);
        --color-surface-hover: var(--color-surface-hover-dark);
        --color-border: var(--color-border-dark);
        --color-text: var(--color-text-dark);
        --color-text-muted: var(--color-text-muted-dark);
        --color-header: var(--color-header-dark);
        --color-card: var(--color-card-dark);
        --color-card-hover: var(--color-card-hover-dark);
        --color-shadow: rgba(0, 0, 0, 0.3);
    }
}

[data-theme="dark"] {
    --color-background: var(--color-background-dark);
    --color-surface: var(--color-surface-dark);
    --color-surface-hover: var(--color-surface-hover-dark);
    --color-border: var(--color-border-dark);
    --color-text: var(--color-text-dark);
    --color-text-muted: var(--color-text-muted-dark);
    --color-header: var(--color-header-dark);
    --color-card: var(--color-card-dark);
    --color-card-hover: var(--color-card-hover-dark);
    --color-shadow: rgba(0, 0, 0, 0.3);
}

/* Reduced motion */
@media (prefers-reduced-motion: reduce) {
    * {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
    }
}

/* Base styles */
body {
    font-family: var(--font-family);
    font-size: var(--font-size-base);
    line-height: 1.3333;
    color: var(--color-text);
    background-color: var(--color-background);
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

/* Layout */
#app {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    background-color: var(--color-background);
}

/* Header */
.app-header {
    background-color: var(--color-header);
    border-bottom: 1px solid var(--color-border);
    position: sticky;
    top: 0;
    z-index: 1000;
    box-shadow: var(--shadow);
    padding: 0 var(--space-4);
}

.nav {
    display: flex;
    align-items: center;
    justify-content: space-between;
    height: 56px;
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 var(--space-4);
}

.nav-title {
    font-size: var(--font-size-xl);
    font-weight: var(--font-weight-bold);
    color: var(--color-primary);
    text-decoration: none;
    display: flex;
    align-items: center;
    gap: var(--space-2);
}

.nav-actions {
    display: flex;
    align-items: center;
    gap: var(--space-2);
}

/* Main content */
.main-content {
    flex: 1;
    padding: var(--space-6) var(--space-4);
    max-width: 1200px;
    margin: 0 auto;
    width: 100%;
}

/* Cards */
.card {
    background: var(--color-card);
    border-radius: var(--border-radius);
    border: 1px solid var(--color-border);
    box-shadow: var(--shadow-card);
    transition: all 0.2s ease-in-out;
    overflow: hidden;
}

.card:hover {
    box-shadow: var(--shadow-md);
    transform: translateY(-1px);
}

.card-header {
    padding: var(--space-4);
    border-bottom: 1px solid var(--color-border);
    font-size: var(--font-size-lg);
    font-weight: var(--font-weight-bold);
}

.card-body {
    padding: var(--space-4);
}

.card-footer {
    padding: var(--space-4);
    border-top: 1px solid var(--color-border);
    background: var(--color-surface);
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: var(--space-2);
    padding: 0 16px;
    height: 36px;
    border: none;
    border-radius: var(--border-radius);
    font-family: var(--font-family);
    font-size: var(--font-size-base);
    font-weight: var(--font-weight-medium);
    text-decoration: none;
    cursor: pointer;
    transition: all 0.2s cubic-bezier(.08,.52,.52,1);
    white-space: nowrap;
    user-select: none;
    -webkit-font-smoothing: antialiased;
}

.btn:disabled {
    opacity: 0.6;
    cursor: not-allowed;
}

.btn:focus {
    outline: none;
    box-shadow: 0 0 0 2px rgba(24, 119, 242, 0.2);
}

.btn-sm {
    height: 28px;
    padding: 0 12px;
    font-size: var(--font-size-sm);
}

.btn-lg {
    height: 44px;
    padding: 0 24px;
    font-size: var(--font-size-md);
}

/* Primary Button */
.btn-primary {
    background-color: var(--color-primary);
    color: var(--color-text-inverse);
    font-weight: var(--font-weight-bold);
}

.btn-primary:not(:disabled):hover {
    background-color: var(--color-primary-hover);
    transform: translateY(-1px);
}

.btn-primary:not(:disabled):active {
    transform: translateY(0);
}

/* Secondary Button */
.btn-secondary {
    background-color: var(--color-secondary);
    color: var(--color-text-inverse);
}

.btn-secondary:not(:disabled):hover {
    background-color: #d8dadf;
    transform: translateY(-1px);
}

.btn-secondary:not(:disabled):active {
    transform: translateY(0);
}

/* Ghost Button */
.btn-ghost {
    background-color: transparent;
    color: var(--color-primary);
    border: 1px solid var(--color-primary);
}

.btn-ghost:not(:disabled):hover {
    background-color: rgba(24, 119, 242, 0.1);
    transform: translateY(-1px);
}

/* Icon Button */
.btn-icon {
    width: 36px;
    height: 36px;
    padding: 0;
    border-radius: 50%;
    background-color: var(--color-surface);
    color: var(--color-text);
}

.btn-icon:hover {
    background-color: var(--color-surface-hover);
}

/* Button with Icon */
.btn .icon {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 20px;
    height: 20px;
}

.btn-ghost {
    background-color: transparent;
    color: var(--color-text-muted);
    border-color: var(--color-border);
}

.btn-ghost:hover {
    background-color: var(--color-surface-hover);
    color: var(--color-text);
}

.btn-success {
    background-color: var(--color-success);
    color: var(--color-text-inverse);
}

.btn-success:hover {
    background-color: var(--color-success-hover);
}

.btn-warning {
    background-color: var(--color-warning);
    color: var(--color-text-inverse);
}

.btn-warning:hover {
    background-color: var(--color-warning-hover);
}

.btn-error {
    background-color: var(--color-error);
    color: var(--color-text-inverse);
}

.btn-error:hover {
    background-color: var(--color-error-hover);
}

.btn-lg {
    padding: var(--space-4) var(--space-6);
    font-size: var(--font-size-base);
}

.btn-sm {
    padding: var(--space-2) var(--space-3);
    font-size: var(--font-size-xs);
}

/* Cards */
.card {
    background-color: var(--color-surface);
    border: var(--border-width) solid var(--color-border);
    border-radius: var(--border-radius-lg);
    padding: var(--space-6);
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}

.card:hover {
    box-shadow: var(--shadow-md);
}

.card-header {
    margin-bottom: var(--space-4);
}

.card-title {
    font-size: var(--font-size-lg);
    font-weight: 600;
    margin-bottom: var(--space-2);
}

.card-description {
    color: var(--color-text-muted);
    font-size: var(--font-size-sm);
}

/* Deck grid */
.deck-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: var(--space-6);
    margin-top: var(--space-8);
}

.deck-card {
    cursor: pointer;
    position: relative;
}

.deck-card:hover {
    transform: translateY(-2px);
}

.deck-stats {
    display: flex;
    justify-content: space-between;
    margin-top: var(--space-4);
    font-size: var(--font-size-sm);
    color: var(--color-text-muted);
}

.deck-due {
    color: var(--color-primary);
    font-weight: 500;
}

/* Study interface */
.study-container {
    max-width: 800px;
    margin: 0 auto;
}

.study-hud {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: var(--space-8);
    padding: var(--space-4);
    background-color: var(--color-surface);
    border-radius: var(--border-radius);
    border: var(--border-width) solid var(--color-border);
}

.study-progress {
    display: flex;
    gap: var(--space-4);
    font-size: var(--font-size-sm);
    color: var(--color-text-muted);
}

.flashcard {
    background-color: var(--color-surface);
    border: var(--border-width) solid var(--color-border);
    border-radius: var(--border-radius-lg);
    min-height: 400px;
    display: flex;
    align-items: center;
    justify-content: center;
    text-align: center;
    padding: var(--space-8);
    margin-bottom: var(--space-8);
    cursor: pointer;
    transition: var(--transition);
    box-shadow: var(--shadow);
}

.flashcard:hover {
    box-shadow: var(--shadow-lg);
}

.flashcard-content {
    font-size: var(--font-size-2xl);
    line-height: 1.4;
}

.flashcard.flipped {
    background-color: var(--color-primary);
    color: var(--color-text-inverse);
}

.grade-buttons {
    display: flex;
    gap: var(--space-4);
    justify-content: center;
    flex-wrap: wrap;
}

.grade-btn {
    flex: 1 1 180px;
    /* Allow labels to wrap and avoid truncation */
    min-width: 180px;
    height: auto; /* override .btn fixed height */
    min-height: 72px;
    display: flex;
    flex-direction: column;
    align-items: center;
    text-align: center;
    white-space: normal;
    line-height: 1.2;
    gap: var(--space-1);
    padding: var(--space-5);
}

.grade-number {
    font-size: var(--font-size-xl);
    font-weight: 600;
}

.grade-label {
    font-size: var(--font-size-sm);
    opacity: 0.9;
}

/* Modals */
.modal-overlay {
    position: fixed;
    inset: 0;
    background-color: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 1000;
    padding: var(--space-4);
}

.modal-overlay.hidden {
    display: none;
}

.modal-content {
    background-color: var(--color-background);
    border-radius: var(--border-radius-lg);
    padding: var(--space-6);
    max-width: 500px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
    box-shadow: var(--shadow-lg);
}

.modal-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: var(--space-4);
}

.modal-title {
    font-size: var(--font-size-lg);
    font-weight: 600;
}

.modal-close {
    background: none;
    border: none;
    font-size: var(--font-size-xl);
    cursor: pointer;
    color: var(--color-text-muted);
    padding: var(--space-1);
}

/* Toast notifications */
.toast-container {
    position: fixed;
    top: var(--space-4);
    right: var(--space-4);
    z-index: 1100;
    display: flex;
    flex-direction: column;
    gap: var(--space-2);
}

.toast {
    background-color: var(--color-surface);
    border: var(--border-width) solid var(--color-border);
    border-radius: var(--border-radius);
    padding: var(--space-4);
    box-shadow: var(--shadow-lg);
    max-width: 400px;
    animation: slideIn 0.3s ease-out;
}

.toast.success {
    border-color: var(--color-success);
    background-color: var(--color-success);
    color: var(--color-text-inverse);
}

.toast.error {
    border-color: var(--color-error);
    background-color: var(--color-error);
    color: var(--color-text-inverse);
}

@keyframes slideIn {
    from {
        transform: translateX(100%);
        opacity: 0;
    }
    to {
        transform: translateX(0);
        opacity: 1;
    }
}

/* Forms */
.form-group {
    margin-bottom: var(--space-4);
}

.form-label {
    display: block;
    margin-bottom: var(--space-2);
    font-weight: 500;
    font-size: var(--font-size-sm);
}

.form-input,
.form-textarea,
.form-select {
    width: 100%;
    padding: var(--space-3);
    border: var(--border-width) solid var(--color-border);
    border-radius: var(--border-radius);
    font-size: var(--font-size-base);
    background-color: var(--color-background);
    color: var(--color-text);
    transition: var(--transition);
}

.form-input:focus,
.form-textarea:focus,
.form-select:focus {
    outline: none;
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.1);
}

.form-textarea {
    resize: vertical;
    min-height: 100px;
}

/* Utility classes */
.hidden {
    display: none !important;
}

.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border: 0;
}

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

.text-muted {
    color: var(--color-text-muted);
}

.mb-4 {
    margin-bottom: var(--space-4);
}

.mt-8 {
    margin-top: var(--space-8);
}

/* Additional utility classes (for CSP-compliant styling) */
.mt-2 { margin-top: var(--space-2); }
.mt-4 { margin-top: var(--space-4); }
.mb-2 { margin-bottom: var(--space-2); }
.mb-6 { margin-bottom: var(--space-6); }
.p-4 { padding: var(--space-4); }

.flex { display: flex; }
.flex-wrap { flex-wrap: wrap; }
.items-center { align-items: center; }
.items-start { align-items: flex-start; }
.justify-between { justify-content: space-between; }
.justify-center { justify-content: center; }
.justify-end { justify-content: flex-end; }
.gap-1 { gap: var(--space-1); }
.gap-2 { gap: var(--space-2); }
.gap-4 { gap: var(--space-4); }

.text-xs { font-size: var(--font-size-xs); }
.text-sm { font-size: var(--font-size-sm); }
.text-2xl { font-size: var(--font-size-2xl); }
.text-6xl { font-size: 4rem; }
.font-semibold { font-weight: 600; }

.text-primary { color: var(--color-primary); }
.text-success { color: var(--color-success); }
.text-warning { color: var(--color-warning); }
.opacity-80 { opacity: 0.8; }

/* Component-specific helpers */
.deck-stats-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: var(--space-4);
}

.session-stats {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: var(--space-4);
    max-width: 600px;
    margin: var(--space-8) auto;
}

.flashcard-hint {
    margin-top: var(--space-4);
    font-size: var(--font-size-sm);
    color: var(--color-text-muted);
}

.tag-pill {
    padding: var(--space-1) var(--space-2);
    background: var(--color-surface-hover);
    border-radius: var(--border-radius);
    font-size: var(--font-size-sm);
}

/* SVG progress ring transition */
.progress-ring-value {
    transition: stroke-dashoffset 0.3s ease;
}

/* File drop zone */
.file-drop-zone {
    border: 2px dashed var(--color-border);
    border-radius: var(--border-radius-lg);
    padding: var(--space-12);
    text-align: center;
    color: var(--color-text-muted);
    transition: var(--transition);
    cursor: pointer;
}

.file-drop-zone:hover,
.file-drop-zone.drag-over {
    border-color: var(--color-primary);
    background-color: rgba(37, 99, 235, 0.05);
    color: var(--color-primary);
}

.file-drop-zone-icon {
    font-size: var(--font-size-4xl);
    margin-bottom: var(--space-4);
}

/* Responsive design */
@media (max-width: 768px) {
    .nav {
        padding: var(--space-3) var(--space-4);
    }
    
    .main-content {
        padding: var(--space-4);
    }
    
    .deck-grid {
        grid-template-columns: 1fr;
        gap: var(--space-4);
    }
    
    .flashcard {
        min-height: 300px;
        padding: var(--space-6);
    }
    
    .flashcard-content {
        font-size: var(--font-size-xl);
    }
    
    .grade-buttons {
        gap: var(--space-2);
        flex-wrap: wrap;
    }
    
    .grade-btn {
        padding: var(--space-4);
        min-width: 160px;
        min-height: 64px;
        font-size: var(--font-size-lg);
    }
    
    .modal-content {
        margin: var(--space-4);
        max-width: calc(100vw - 2rem);
    }
}

/* Empty state icon */
.empty-state-icon {
    display: inline-block;
}

.empty-state-icon svg {
    width: 96px;
    height: 96px;
    max-width: 50vw;
    max-height: 50vw;
}

/* Ensure strokes render crisply across themes */
.empty-state-icon .card-stroke {
    shape-rendering: geometricPrecision;
}

/* Allow slight hover emphasis if interactive in future */
.empty-state-icon:hover {
    filter: drop-shadow(0 1px 2px rgba(0,0,0,0.06));
}