/* Mobile-first CSS Variables */
:root {
    --color-primary: #4CAF50;
    --color-primary-dark: #388E3C;
    --color-secondary: #2196F3;
    --color-correct: #81C784;
    --color-correct-bg: #E8F5E9;
    --color-incorrect: #E57373;
    --color-incorrect-bg: #FFEBEE;
    --color-bg: #FAFAFA;
    --color-card: #FFFFFF;
    --color-text: #333333;
    --color-text-light: #757575;
    --color-border: #E0E0E0;
    --shadow-card: 0 2px 8px rgba(0, 0, 0, 0.1);
    --shadow-button: 0 2px 4px rgba(0, 0, 0, 0.15);
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --transition-speed: 0.3s;
}

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

/* Base */
html {
    font-size: 16px;
    -webkit-text-size-adjust: 100%;
}

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

/* Screens */
.screen {
    display: none;
    flex-direction: column;
    min-height: 100vh;
    min-height: 100dvh;
    padding: 16px;
    padding-bottom: env(safe-area-inset-bottom, 16px);
}

.screen.active {
    display: flex;
}

/* Header */
.header {
    text-align: center;
    padding: 24px 0 16px;
}

.header h1 {
    font-size: 1.75rem;
    margin-bottom: 4px;
}

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

/* Header Nav */
.header-nav {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 12px 0;
    margin-bottom: 8px;
}

.header-nav h2 {
    font-size: 1.25rem;
    flex: 1;
    text-align: center;
}

.btn-back {
    width: 44px;
    height: 44px;
    border: none;
    background: var(--color-card);
    border-radius: var(--radius-sm);
    font-size: 1.25rem;
    cursor: pointer;
    box-shadow: var(--shadow-button);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.btn-back:active {
    transform: scale(0.95);
}

.progress-badge {
    min-width: 44px;
    padding: 4px 8px;
    background: var(--color-primary);
    color: white;
    border-radius: var(--radius-sm);
    font-size: 0.875rem;
    font-weight: 600;
    text-align: center;
}

/* Categories Grid */
.categories-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
    flex: 1;
    padding: 8px 0;
}

.category-card {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 20px 12px;
    background: var(--color-card);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-card);
    cursor: pointer;
    transition: transform var(--transition-speed), box-shadow var(--transition-speed);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
    border: 2px solid transparent;
}

.category-card:active {
    transform: scale(0.97);
}

.category-card.completed {
    border-color: var(--color-primary);
}

.category-emoji {
    font-size: 3rem;
    margin-bottom: 8px;
}

.category-name {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: 4px;
}

.category-count {
    font-size: 0.75rem;
    color: var(--color-text-light);
}

.category-bar {
    width: 100%;
    height: 4px;
    background: var(--color-border);
    border-radius: 2px;
    margin-top: 8px;
    overflow: hidden;
}

.category-bar-fill {
    height: 100%;
    background: var(--color-primary);
    border-radius: 2px;
    transition: width var(--transition-speed);
}

/* Stats Bar */
.stats-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 12px 16px;
    background: var(--color-card);
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-card);
    margin-top: auto;
}

.stat {
    display: flex;
    align-items: center;
    gap: 4px;
    font-size: 0.875rem;
    font-weight: 500;
}

.stat-icon {
    font-size: 1.25rem;
}

.btn-icon {
    width: 44px;
    height: 44px;
    border: none;
    background: transparent;
    font-size: 1.5rem;
    cursor: pointer;
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

/* Category Menu */
.menu-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 24px 0;
}

.menu-emoji {
    font-size: 5rem;
    margin-bottom: 24px;
}

.menu-description {
    color: var(--color-text-light);
    margin-bottom: 32px;
}

.menu-buttons {
    width: 100%;
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.btn-menu {
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 20px;
    background: var(--color-card);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-lg);
    cursor: pointer;
    transition: all var(--transition-speed);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.btn-menu:active {
    transform: scale(0.98);
    border-color: var(--color-primary);
}

.btn-icon-large {
    font-size: 2.5rem;
    margin-bottom: 8px;
}

.btn-label {
    font-size: 1.25rem;
    font-weight: 600;
}

.btn-desc {
    font-size: 0.875rem;
    color: var(--color-text-light);
}

/* Flashcard */
.flashcard-container {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 16px 0;
}

.flashcard {
    width: 100%;
    max-width: 320px;
    background: var(--color-card);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-card);
    padding: 32px 24px;
    text-align: center;
    transition: transform var(--transition-speed);
}

.flashcard-emoji {
    font-size: 6rem;
    line-height: 1.1;
    margin-bottom: 16px;
}

.flashcard-word {
    font-size: 2rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 2px;
    margin-bottom: 8px;
}

.flashcard-translation {
    font-size: 1.25rem;
    color: var(--color-text-light);
    min-height: 1.5em;
    opacity: 0;
    transition: opacity var(--transition-speed);
}

.flashcard-translation.visible {
    opacity: 1;
}

.flashcard-actions {
    display: flex;
    gap: 12px;
    margin-top: 24px;
}

.btn-action {
    flex: 1;
    padding: 14px 20px;
    background: var(--color-card);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition-speed);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.btn-action:active {
    transform: scale(0.97);
}

.btn-action.btn-primary {
    background: var(--color-primary);
    border-color: var(--color-primary);
    color: white;
}

/* Navigation Buttons */
.nav-buttons {
    display: flex;
    gap: 12px;
    padding-top: 16px;
}

.btn-nav {
    flex: 1;
    padding: 16px;
    background: var(--color-card);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all var(--transition-speed);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.btn-nav:active {
    transform: scale(0.97);
}

.btn-nav.btn-primary {
    background: var(--color-primary);
    border-color: var(--color-primary);
    color: white;
}

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

/* Quiz */
.quiz-container {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 16px 0;
}

.quiz-emoji {
    font-size: 6rem;
    margin-bottom: 16px;
}

.quiz-question {
    font-size: 1.25rem;
    color: var(--color-text-light);
    margin-bottom: 24px;
}

.quiz-options {
    width: 100%;
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.quiz-option {
    padding: 18px 24px;
    background: var(--color-card);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    font-size: 1.25rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 1px;
    cursor: pointer;
    transition: all var(--transition-speed);
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.quiz-option:active {
    transform: scale(0.98);
}

.quiz-option.correct {
    background: var(--color-correct-bg);
    border-color: var(--color-correct);
    color: var(--color-primary-dark);
}

.quiz-option.incorrect {
    background: var(--color-incorrect-bg);
    border-color: var(--color-incorrect);
    color: #C62828;
}

.quiz-option.disabled {
    pointer-events: none;
    opacity: 0.6;
}

.quiz-feedback {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 8px;
    padding: 16px;
    border-radius: var(--radius-md);
    font-size: 1.125rem;
    font-weight: 500;
    margin-top: 16px;
    transition: all var(--transition-speed);
}

.quiz-feedback.hidden {
    opacity: 0;
    visibility: hidden;
}

.quiz-feedback.correct {
    background: var(--color-correct-bg);
    color: var(--color-primary-dark);
}

.quiz-feedback.incorrect {
    background: var(--color-incorrect-bg);
    color: #C62828;
}

/* Results */
.results-container {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    text-align: center;
    padding: 24px 0;
}

.results-emoji {
    font-size: 5rem;
    margin-bottom: 16px;
}

.results-container h3 {
    font-size: 1.75rem;
    margin-bottom: 8px;
}

.results-score {
    font-size: 3rem;
    font-weight: 700;
    color: var(--color-primary);
    margin-bottom: 8px;
}

.results-message {
    color: var(--color-text-light);
    margin-bottom: 32px;
}

.results-buttons {
    width: 100%;
    display: flex;
    flex-direction: column;
    gap: 12px;
}

/* Stats Screen */
.stats-container {
    flex: 1;
    padding: 16px 0;
}

.stats-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 12px;
    margin-bottom: 24px;
}

.stat-card {
    display: flex;
    flex-direction: column;
    align-items: center;
    padding: 20px 12px;
    background: var(--color-card);
    border-radius: var(--radius-md);
    box-shadow: var(--shadow-card);
}

.stat-value {
    font-size: 2rem;
    font-weight: 700;
    color: var(--color-primary);
}

.stat-label {
    font-size: 0.75rem;
    color: var(--color-text-light);
    text-align: center;
    margin-top: 4px;
}

.section-title {
    font-size: 1rem;
    margin-bottom: 12px;
}

.category-stats {
    display: flex;
    flex-direction: column;
    gap: 12px;
    margin-bottom: 32px;
}

.category-stat-row {
    display: flex;
    align-items: center;
    gap: 12px;
    padding: 12px;
    background: var(--color-card);
    border-radius: var(--radius-sm);
}

.category-stat-emoji {
    font-size: 1.5rem;
}

.category-stat-info {
    flex: 1;
}

.category-stat-name {
    font-size: 0.875rem;
    font-weight: 500;
}

.category-stat-bar {
    height: 6px;
    background: var(--color-border);
    border-radius: 3px;
    margin-top: 4px;
    overflow: hidden;
}

.category-stat-fill {
    height: 100%;
    background: var(--color-primary);
    border-radius: 3px;
}

.category-stat-count {
    font-size: 0.75rem;
    color: var(--color-text-light);
}

.btn-danger {
    width: 100%;
    padding: 14px;
    background: transparent;
    border: 2px solid var(--color-incorrect);
    border-radius: var(--radius-md);
    color: var(--color-incorrect);
    font-size: 1rem;
    cursor: pointer;
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

/* Modal */
.modal-overlay {
    position: fixed;
    inset: 0;
    background: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 24px;
    opacity: 0;
    visibility: hidden;
    transition: all var(--transition-speed);
    z-index: 100;
}

.modal-overlay.visible {
    opacity: 1;
    visibility: visible;
}

.modal {
    background: var(--color-card);
    border-radius: var(--radius-lg);
    padding: 24px;
    max-width: 360px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
}

.help-modal h2 {
    text-align: center;
    margin-bottom: 24px;
}

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

.help-section h3 {
    font-size: 1rem;
    margin-bottom: 8px;
}

.help-section p {
    font-size: 0.875rem;
    color: var(--color-text-light);
    line-height: 1.5;
}

.btn-full {
    width: 100%;
    padding: 16px;
    border: none;
    border-radius: var(--radius-md);
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    touch-action: manipulation;
    -webkit-tap-highlight-color: transparent;
}

.btn-primary {
    background: var(--color-primary);
    color: white;
}

.btn-primary:active {
    background: var(--color-primary-dark);
}

/* Toast */
.toast {
    position: fixed;
    bottom: 100px;
    left: 50%;
    transform: translateX(-50%) translateY(20px);
    background: var(--color-text);
    color: white;
    padding: 12px 24px;
    border-radius: var(--radius-md);
    font-size: 0.875rem;
    opacity: 0;
    visibility: hidden;
    transition: all var(--transition-speed);
    z-index: 200;
}

.toast.visible {
    opacity: 1;
    visibility: visible;
    transform: translateX(-50%) translateY(0);
}

/* Animations */
@keyframes bounce {
    0%, 100% { transform: scale(1); }
    50% { transform: scale(1.05); }
}

.bounce {
    animation: bounce 0.4s ease-in-out;
}

@keyframes shake {
    0%, 100% { transform: translateX(0); }
    25% { transform: translateX(-8px); }
    75% { transform: translateX(8px); }
}

.shake {
    animation: shake 0.3s ease-in-out;
}

/* Desktop */
@media (min-width: 768px) {
    .screen {
        max-width: 480px;
        margin: 0 auto;
    }

    .categories-grid {
        grid-template-columns: repeat(3, 1fr);
    }

    .category-card:hover {
        transform: translateY(-2px);
        box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
    }

    .btn-menu:hover {
        border-color: var(--color-primary);
        transform: translateY(-2px);
    }

    .quiz-option:hover {
        border-color: var(--color-secondary);
    }
}