/* ===========================
   CSS Variables & Theme
   =========================== */
:root {
    /* Light Theme Colors */
    --bg-primary: #f8f9fa;
    --bg-secondary: #ffffff;
    --bg-tertiary: #e9ecef;
    --text-primary: #212529;
    --text-secondary: #6c757d;
    --text-tertiary: #adb5bd;
    --border-color: #dee2e6;
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.07);
    --shadow-lg: 0 10px 25px rgba(0, 0, 0, 0.1);
    --shadow-xl: 0 20px 40px rgba(0, 0, 0, 0.15);

    /* Brand Colors */
    --color-primary: #0096FF;
    --color-primary-dark: #0F52BA;
    --color-primary-light: rgba(0, 150, 255, 0.1);
    --color-primary-hover: rgba(0, 150, 255, 0.9);

    /* Status Colors */
    --color-success: #28a745;
    --color-success-light: rgba(40, 167, 69, 0.1);
    --color-danger: #dc3545;
    --color-danger-light: rgba(220, 53, 69, 0.1);
    --color-warning: #ffc107;
    --color-warning-light: rgba(255, 193, 7, 0.1);

    /* Priority Colors */
    --priority-low: #28a745;
    --priority-medium: #ffc107;
    --priority-high: #dc3545;

    /* Spacing */
    --spacing-xs: 0.25rem;
    --spacing-sm: 0.5rem;
    --spacing-md: 1rem;
    --spacing-lg: 1.5rem;
    --spacing-xl: 2rem;
    --spacing-2xl: 3rem;

    /* Border Radius */
    --radius-sm: 0.375rem;
    --radius-md: 0.5rem;
    --radius-lg: 0.75rem;
    --radius-xl: 1rem;
    --radius-2xl: 1.5rem;

    /* Transitions */
    --transition-fast: 150ms ease;
    --transition-normal: 250ms ease;
    --transition-slow: 350ms ease;
}

[data-theme="dark"] {
    --bg-primary: #0d1117;
    --bg-secondary: #161b22;
    --bg-tertiary: #21262d;
    --text-primary: #e6edf3;
    --text-secondary: #8b949e;
    --text-tertiary: #6e7681;
    --border-color: #30363d;
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.3);
    --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.4);
    --shadow-lg: 0 10px 25px rgba(0, 0, 0, 0.5);
    --shadow-xl: 0 20px 40px rgba(0, 0, 0, 0.6);
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    transition: background-color var(--transition-normal), color var(--transition-normal);
}

/* ===========================
   Header
   =========================== */
.app-header {
    background: var(--bg-secondary);
    border-bottom: 1px solid var(--border-color);
    padding: var(--spacing-md) var(--spacing-xl);
    position: sticky;
    top: 0;
    z-index: 100;
    box-shadow: var(--shadow-sm);
}

.header-content {
    max-width: 1600px;
    margin: 0 auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.header-left {
    display: flex;
    align-items: center;
    gap: var(--spacing-lg);
}

.portfolio-link {
    color: var(--text-secondary);
    text-decoration: none;
    font-weight: 500;
    transition: color var(--transition-fast);
    padding: var(--spacing-sm) var(--spacing-md);
    border-radius: var(--radius-md);
}

.portfolio-link:hover {
    color: var(--color-primary);
    background: var(--color-primary-light);
}

.logo {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.logo-icon {
    width: 32px;
    height: 32px;
    color: var(--color-primary);
}

.logo h1 {
    font-size: 1.5rem;
    font-weight: 700;
    background: linear-gradient(135deg, var(--color-primary), var(--color-primary-dark));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.header-right {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
}

.theme-toggle {
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.theme-toggle:hover {
    background: var(--bg-primary);
    border-color: var(--color-primary);
}

.theme-toggle svg {
    width: 20px;
    height: 20px;
    color: var(--text-primary);
}

.sun-icon {
    display: none;
}

.moon-icon {
    display: block;
}

[data-theme="dark"] .sun-icon {
    display: block;
}

[data-theme="dark"] .moon-icon {
    display: none;
}

/* ===========================
   Main Layout
   =========================== */
.app-main {
    padding: var(--spacing-xl);
}

.app-container {
    max-width: 1600px;
    margin: 0 auto;
    display: grid;
    grid-template-columns: 320px 1fr;
    gap: var(--spacing-xl);
}

/* ===========================
   Sidebar
   =========================== */
.sidebar {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-lg);
}

.section-title {
    font-size: 0.875rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--text-secondary);
    margin-bottom: var(--spacing-md);
}

/* Stats Section */
.stats-section {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
}

.stats-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: var(--spacing-md);
}

.stat-card {
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    transition: all var(--transition-fast);
}

.stat-card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

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

.stat-content {
    flex: 1;
}

.stat-value {
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--color-primary);
    line-height: 1;
}

.stat-label {
    font-size: 0.75rem;
    color: var(--text-secondary);
    margin-top: var(--spacing-xs);
}

/* Filters Section */
.filters-section {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
}

.filter-buttons {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.filter-btn {
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    cursor: pointer;
    transition: all var(--transition-fast);
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--text-primary);
}

.filter-btn:hover {
    background: var(--bg-tertiary);
    border-color: var(--color-primary);
}

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

.filter-icon {
    font-size: 1.125rem;
}

.filter-count {
    margin-left: auto;
    background: var(--bg-tertiary);
    padding: 0.125rem 0.5rem;
    border-radius: var(--radius-sm);
    font-size: 0.75rem;
    font-weight: 600;
}

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

/* Actions Section */
.actions-section {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
}

.action-btn {
    width: 100%;
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    cursor: pointer;
    transition: all var(--transition-fast);
    font-size: 0.9375rem;
    font-weight: 500;
    color: var(--text-primary);
    margin-bottom: var(--spacing-sm);
}

.action-btn:last-child {
    margin-bottom: 0;
}

.action-btn:hover {
    background: var(--color-danger-light);
    border-color: var(--color-danger);
    color: var(--color-danger);
}

.action-btn svg {
    width: 18px;
    height: 18px;
}

/* ===========================
   Task Area
   =========================== */
.task-area {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-lg);
}

/* Add Task Section */
.add-task-section {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
}

.add-task-form {
    width: 100%;
}

.form-content {
    display: flex;
    gap: var(--spacing-md);
    align-items: stretch;
}

.task-input {
    flex: 1;
    background: var(--bg-primary);
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: var(--spacing-md) var(--spacing-lg);
    font-size: 1rem;
    color: var(--text-primary);
    transition: all var(--transition-fast);
}

.task-input:focus {
    outline: none;
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px var(--color-primary-light);
}

.priority-select {
    background: var(--bg-primary);
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: var(--spacing-md) var(--spacing-lg);
    font-size: 0.9375rem;
    color: var(--text-primary);
    cursor: pointer;
    transition: all var(--transition-fast);
    min-width: 150px;
}

.priority-select:focus {
    outline: none;
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px var(--color-primary-light);
}

.add-btn {
    background: var(--color-primary);
    border: none;
    border-radius: var(--radius-md);
    padding: var(--spacing-md) var(--spacing-xl);
    color: white;
    font-size: 0.9375rem;
    font-weight: 600;
    cursor: pointer;
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    transition: all var(--transition-fast);
    white-space: nowrap;
}

.add-btn:hover {
    background: var(--color-primary-hover);
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}

.add-btn svg {
    width: 18px;
    height: 18px;
}

/* Task List Container */
.task-list-container {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-xl);
    padding: var(--spacing-lg);
    box-shadow: var(--shadow-sm);
    min-height: 500px;
}

.task-list-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: var(--spacing-lg);
    padding-bottom: var(--spacing-md);
    border-bottom: 1px solid var(--border-color);
}

.current-filter-title {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--text-primary);
}

.sort-options {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

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

.sort-select {
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    padding: var(--spacing-sm) var(--spacing-md);
    font-size: 0.875rem;
    color: var(--text-primary);
    cursor: pointer;
}

/* Task List */
.task-list {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-md);
}

.task-item {
    background: var(--bg-primary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-lg);
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    transition: all var(--transition-normal);
    cursor: move;
}

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

.task-item.completed {
    opacity: 0.6;
}

.task-item.completed .task-text {
    text-decoration: line-through;
    color: var(--text-secondary);
}

.task-item.dragging {
    opacity: 0.5;
    transform: rotate(2deg);
}

.task-checkbox {
    width: 24px;
    height: 24px;
    border: 2px solid var(--border-color);
    border-radius: var(--radius-sm);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all var(--transition-fast);
    flex-shrink: 0;
}

.task-checkbox:hover {
    border-color: var(--color-primary);
    background: var(--color-primary-light);
}

.task-checkbox.checked {
    background: var(--color-primary);
    border-color: var(--color-primary);
}

.task-checkbox svg {
    width: 16px;
    height: 16px;
    color: white;
    display: none;
}

.task-checkbox.checked svg {
    display: block;
}

.task-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: var(--spacing-xs);
}

.task-text {
    font-size: 1rem;
    color: var(--text-primary);
    word-break: break-word;
}

.task-meta {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    font-size: 0.75rem;
    color: var(--text-secondary);
}

.task-priority {
    display: inline-flex;
    align-items: center;
    gap: 0.25rem;
    padding: 0.125rem 0.5rem;
    border-radius: var(--radius-sm);
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.025em;
}

.task-priority.low {
    background: var(--color-success-light);
    color: var(--priority-low);
}

.task-priority.medium {
    background: var(--color-warning-light);
    color: var(--priority-medium);
}

.task-priority.high {
    background: var(--color-danger-light);
    color: var(--priority-high);
}

.task-time {
    display: flex;
    align-items: center;
    gap: 0.25rem;
}

.task-actions {
    display: flex;
    gap: var(--spacing-sm);
}

.task-action-btn {
    width: 36px;
    height: 36px;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-md);
    display: flex;
    align-items: center;
    justify-content: center;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.task-action-btn:hover {
    transform: scale(1.1);
}

.task-action-btn.edit:hover {
    background: var(--color-primary-light);
    border-color: var(--color-primary);
    color: var(--color-primary);
}

.task-action-btn.delete:hover {
    background: var(--color-danger-light);
    border-color: var(--color-danger);
    color: var(--color-danger);
}

.task-action-btn svg {
    width: 18px;
    height: 18px;
}

/* Empty State */
.empty-state {
    display: none;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: var(--spacing-2xl);
    text-align: center;
    min-height: 400px;
}

.empty-state.visible {
    display: flex;
}

.empty-icon {
    width: 80px;
    height: 80px;
    color: var(--text-tertiary);
    margin-bottom: var(--spacing-lg);
}

.empty-state h3 {
    font-size: 1.5rem;
    color: var(--text-secondary);
    margin-bottom: var(--spacing-sm);
}

.empty-state p {
    color: var(--text-tertiary);
}

/* ===========================
   Toast Notifications
   =========================== */
.toast-container {
    position: fixed;
    bottom: var(--spacing-xl);
    right: var(--spacing-xl);
    z-index: 1000;
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.toast {
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-lg);
    padding: var(--spacing-md) var(--spacing-lg);
    box-shadow: var(--shadow-xl);
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    min-width: 300px;
    animation: slideIn 0.3s ease;
}

.toast.success {
    border-left: 4px solid var(--color-success);
}

.toast.error {
    border-left: 4px solid var(--color-danger);
}

.toast.info {
    border-left: 4px solid var(--color-primary);
}

.toast-icon {
    font-size: 1.5rem;
}

.toast-content {
    flex: 1;
}

.toast-title {
    font-weight: 600;
    margin-bottom: 0.125rem;
}

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

@keyframes slideIn {
    from {
        transform: translateX(400px);
        opacity: 0;
    }
    to {
        transform: translateX(0);
        opacity: 1;
    }
}

/* ===========================
   Responsive Design
   =========================== */
@media (max-width: 1200px) {
    .app-container {
        grid-template-columns: 280px 1fr;
    }
}

@media (max-width: 968px) {
    .app-container {
        grid-template-columns: 1fr;
    }

    .sidebar {
        order: 2;
    }

    .task-area {
        order: 1;
    }

    .stats-grid {
        grid-template-columns: repeat(4, 1fr);
    }

    .filters-section,
    .actions-section {
        display: none;
    }
}

@media (max-width: 640px) {
    .app-main {
        padding: var(--spacing-md);
    }

    .app-header {
        padding: var(--spacing-md);
    }

    .header-left {
        gap: var(--spacing-md);
    }

    .logo h1 {
        font-size: 1.25rem;
    }

    .form-content {
        flex-direction: column;
    }

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

    .task-list-header {
        flex-direction: column;
        align-items: flex-start;
        gap: var(--spacing-md);
    }

    .toast-container {
        left: var(--spacing-md);
        right: var(--spacing-md);
    }

    .toast {
        min-width: auto;
    }
}