/* ===== Navigation ===== */
.nav-bar {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.75rem 1.5rem;
    background: var(--white);
    border-bottom: 1px solid var(--gray-200);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav-brand {
    flex-shrink: 0;
}

.nav-logo {
    font-family: 'Noto Sans SC', sans-serif;
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--gray-900);
    text-decoration: none;
}

.nav-logo span {
    font-family: 'Inter', sans-serif;
    color: var(--primary);
    font-weight: 600;
}

.nav-links {
    display: flex;
    gap: 0.25rem;
}

.nav-link {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.5rem 1rem;
    color: var(--gray-500);
    text-decoration: none;
    border-radius: var(--radius);
    font-size: 0.875rem;
    font-weight: 500;
    transition: var(--transition);
}

.nav-link:hover {
    color: var(--gray-700);
    background: var(--gray-100);
}

.nav-link.active {
    color: var(--primary);
    background: var(--primary-bg);
}

.nav-actions {
    flex-shrink: 0;
}

/* ===== Main Layout ===== */
.app-main {
    min-height: calc(100vh - 57px);
}

.screen {
    padding: 1.5rem;
    max-width: 1200px;
    margin: 0 auto;
}

/* ===== Home Screen ===== */
.home-container {
    max-width: 960px;
    margin: 0 auto;
}

.home-header {
    text-align: center;
    padding: 2rem 0;
}

.home-title {
    font-family: 'Noto Sans SC', 'Inter', sans-serif;
    font-size: 2.5rem;
    font-weight: 700;
    color: var(--gray-900);
    margin-bottom: 0.5rem;
}

.home-subtitle {
    color: var(--gray-500);
    font-size: 1.125rem;
}

/* ===== Stats Row ===== */
.stats-row {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 1rem;
    margin-bottom: 2rem;
}

.stat-card {
    background: var(--white);
    border: 1px solid var(--gray-200);
    border-radius: var(--radius-lg);
    padding: 1.25rem;
    text-align: center;
    transition: var(--transition);
}

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

.stat-card__value {
    font-size: 2rem;
    font-weight: 700;
    color: var(--gray-500);
    line-height: 1;
    margin-bottom: 0.25rem;
}

.stat-card--learning .stat-card__value { color: var(--info); }
.stat-card--known .stat-card__value { color: var(--warning); }
.stat-card--mastered .stat-card__value { color: var(--success); }

.stat-card__label {
    font-size: 0.8125rem;
    color: var(--gray-500);
    font-weight: 500;
}

/* ===== Section Titles ===== */
.section-title {
    font-size: 1.5rem;
    font-weight: 600;
    color: var(--gray-900);
    margin-bottom: 1rem;
}

/* ===== Lesson Grid ===== */
.lessons-section {
    margin-bottom: 2rem;
}

.lesson-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
    gap: 1rem;
}

.lesson-card {
    display: block;
    background: var(--white);
    border: 1px solid var(--gray-200);
    border-radius: var(--radius-lg);
    padding: 1rem;
    text-decoration: none;
    transition: var(--transition);
    cursor: pointer;
}

.lesson-card:hover {
    border-color: var(--primary-light);
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}

.lesson-card__header {
    display: flex;
    align-items: center;
    gap: 0.375rem;
    margin-bottom: 0.375rem;
}

.lesson-card__icon {
    font-size: 1.25rem;
}

.lesson-card__number {
    font-size: 0.6875rem;
    font-weight: 600;
    color: var(--primary);
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.lesson-card__title {
    font-size: 1rem;
    font-weight: 600;
    color: var(--gray-900);
    margin-bottom: 0.25rem;
}

.lesson-card__chars {
    font-family: 'Noto Sans SC', sans-serif;
    font-size: 0.9375rem;
    color: var(--gray-500);
    line-height: 1.6;
    margin-bottom: 0.5rem;
    word-break: break-all;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
}

.lesson-card__meta {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.75rem;
    color: var(--gray-400);
}

/* ===== Browse Screen ===== */
.browse-container {
    max-width: 1100px;
    margin: 0 auto;
}

.browse-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 1rem;
    margin-bottom: 1rem;
    flex-wrap: wrap;
}

.search-box {
    position: relative;
    width: 280px;
}

.search-box i {
    position: absolute;
    left: 0.75rem;
    top: 50%;
    transform: translateY(-50%);
    color: var(--gray-400);
    font-size: 0.875rem;
}

.search-box input {
    width: 100%;
    padding: 0.5rem 0.75rem 0.5rem 2.25rem;
    background: var(--white);
    border: 1px solid var(--gray-300);
    border-radius: var(--radius);
    font-size: 0.875rem;
    color: var(--gray-800);
    transition: var(--transition);
}

.search-box input::placeholder {
    color: var(--gray-400);
}

.search-box input:focus {
    outline: none;
    border-color: var(--primary);
    box-shadow: 0 0 0 3px rgba(var(--primary-rgb), 0.1);
}

/* ===== Filter Bar ===== */
.filter-bar {
    display: flex;
    gap: 0.375rem;
    overflow-x: auto;
    padding-bottom: 0.5rem;
    margin-bottom: 1rem;
    scrollbar-width: none;
}

.filter-bar::-webkit-scrollbar {
    display: none;
}

.filter-btn {
    flex-shrink: 0;
    padding: 0.375rem 0.75rem;
    font-size: 0.8125rem;
    font-weight: 500;
    background: var(--white);
    color: var(--gray-600);
    border: 1px solid var(--gray-200);
    border-radius: var(--radius-full);
    cursor: pointer;
    transition: var(--transition);
    white-space: nowrap;
}

.filter-btn:hover {
    border-color: var(--gray-300);
    background: var(--gray-50);
}

.filter-btn.active {
    background: var(--primary);
    color: var(--on-primary);
    border-color: var(--primary);
}

/* ===== Empty State ===== */
.empty-state {
    text-align: center;
    padding: 3rem 1rem;
    color: var(--gray-400);
}

.empty-state i {
    font-size: 2rem;
    margin-bottom: 0.75rem;
}

.empty-state p {
    font-size: 1rem;
}

/* ===== Modal ===== */
.modal {
    position: fixed;
    inset: 0;
    z-index: 1000;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 1rem;
}

.modal.hidden {
    display: none;
}

.modal-backdrop {
    position: absolute;
    inset: 0;
    background: var(--modal-overlay);
}

.modal-content {
    position: relative;
    background: var(--white);
    border-radius: var(--radius-xl);
    padding: 2rem;
    max-width: 480px;
    width: 100%;
    max-height: 90vh;
    overflow-y: auto;
    box-shadow: var(--shadow-xl);
    animation: slideUp 0.3s ease;
}

.modal-close {
    position: absolute;
    top: 0.75rem;
    right: 0.75rem;
    z-index: 10;
}

/* ===== Lesson Detail Screen ===== */
.lesson-header {
    margin-bottom: 1.5rem;
}

.lesson-header__back {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    color: var(--gray-500);
    text-decoration: none;
    font-size: 0.875rem;
    margin-bottom: 0.75rem;
}

.lesson-header__back:hover {
    color: var(--primary);
}

.quiz-buttons {
    display: flex;
    gap: 0.75rem;
    margin: 1.5rem 0;
    flex-wrap: wrap;
}

.quiz-btn {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.75rem 1.25rem;
    background: var(--white);
    border: 1px solid var(--gray-200);
    border-radius: var(--radius-lg);
    cursor: pointer;
    transition: var(--transition);
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--gray-700);
    text-decoration: none;
}

.quiz-btn:hover {
    border-color: var(--primary);
    color: var(--primary);
    box-shadow: var(--shadow);
}

.quiz-btn i {
    font-size: 1.125rem;
}

/* ===== Responsive ===== */
@media (max-width: 768px) {
    .nav-bar {
        padding: 0.5rem 1rem;
    }

    .nav-link span {
        display: none;
    }

    .nav-link {
        padding: 0.5rem 0.75rem;
    }

    .screen {
        padding: 1rem;
    }

    .home-title {
        font-size: 2rem;
    }

    .stats-row {
        grid-template-columns: repeat(2, 1fr);
    }

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

    .browse-header {
        flex-direction: column;
        align-items: stretch;
    }

    .search-box {
        width: 100%;
    }

    .modal-content {
        padding: 1.5rem;
    }
}

@media (max-width: 480px) {
    .stats-row {
        grid-template-columns: repeat(2, 1fr);
        gap: 0.5rem;
    }

    .stat-card {
        padding: 1rem;
    }

    .stat-card__value {
        font-size: 1.5rem;
    }
}