/* ========================================
   CSS Variables & Theme System
======================================== */
:root {
    /* Light Theme (Default) */
    --bg-primary: #f0f4f8;
    --bg-secondary: #ffffff;
    --bg-tertiary: #e8eef4;
    --bg-glass: rgba(255, 255, 255, 0.7);
    --text-primary: #1a202c;
    --text-secondary: #4a5568;
    --text-muted: #718096;
    --border-color: rgba(0, 0, 0, 0.08);
    --shadow-color: rgba(0, 0, 0, 0.08);
    --shadow-lg: 0 10px 40px rgba(0, 0, 0, 0.1);
    --accent-gradient: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --accent-color: #667eea;
    --accent-hover: #5a67d8;
    
    /* Priority Colors */
    --priority-high: #ef4444;
    --priority-high-bg: rgba(239, 68, 68, 0.1);
    --priority-medium: #f59e0b;
    --priority-medium-bg: rgba(245, 158, 11, 0.1);
    --priority-low: #10b981;
    --priority-low-bg: rgba(16, 185, 129, 0.1);
    
    /* Success/Danger */
    --success-color: #10b981;
    --danger-color: #ef4444;
    
    /* Transitions */
    --transition-fast: 0.15s ease;
    --transition-normal: 0.3s ease;
    --transition-slow: 0.5s ease;
    
    /* Border Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-xl: 24px;
}

/* Dark Theme */
[data-theme="dark"] {
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --bg-tertiary: #334155;
    --bg-glass: rgba(30, 41, 59, 0.8);
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --text-muted: #94a3b8;
    --border-color: rgba(255, 255, 255, 0.08);
    --shadow-color: rgba(0, 0, 0, 0.3);
    --shadow-lg: 0 10px 40px rgba(0, 0, 0, 0.4);
    --accent-gradient: linear-gradient(135deg, #818cf8 0%, #a78bfa 100%);
    --accent-color: #818cf8;
    --accent-hover: #6366f1;
}

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

html {
    scroll-behavior: smooth;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
    transition: background-color var(--transition-normal), color var(--transition-normal);
}

/* ========================================
   App Container
======================================== */
.app-container {
    max-width: 800px;
    margin: 0 auto;
    padding: 20px;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

/* ========================================
   Header
======================================== */
.header {
    margin-bottom: 30px;
}

.header-content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 20px 24px;
    background: var(--bg-glass);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-radius: var(--radius-xl);
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-lg);
}

.logo {
    display: flex;
    align-items: center;
    gap: 12px;
}

.logo-icon {
    width: 48px;
    height: 48px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--accent-gradient);
    color: white;
    font-size: 24px;
    font-weight: 700;
    border-radius: var(--radius-md);
    box-shadow: 0 4px 15px rgba(102, 126, 234, 0.4);
}

.logo h1 {
    font-size: 1.75rem;
    font-weight: 700;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.theme-toggle {
    width: 50px;
    height: 50px;
    border: none;
    background: var(--bg-tertiary);
    border-radius: var(--radius-md);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 1.5rem;
    transition: all var(--transition-normal);
    position: relative;
    overflow: hidden;
}

.theme-toggle:hover {
    transform: scale(1.05);
    box-shadow: 0 4px 15px var(--shadow-color);
}

.theme-icon {
    position: absolute;
    transition: all var(--transition-normal);
}

.theme-icon.sun {
    opacity: 1;
    transform: translateY(0);
}

.theme-icon.moon {
    opacity: 0;
    transform: translateY(30px);
}

[data-theme="dark"] .theme-icon.sun {
    opacity: 0;
    transform: translateY(-30px);
}

[data-theme="dark"] .theme-icon.moon {
    opacity: 1;
    transform: translateY(0);
}

/* ========================================
   Main Content
======================================== */
.main-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 24px;
}

/* ========================================
   Add Task Section
======================================== */
.add-task-section {
    background: var(--bg-glass);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-radius: var(--radius-xl);
    border: 1px solid var(--border-color);
    padding: 24px;
    box-shadow: var(--shadow-lg);
}

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

.input-group {
    display: flex;
    gap: 12px;
    flex-wrap: wrap;
}

.task-input {
    flex: 1;
    min-width: 200px;
    padding: 16px 20px;
    font-size: 1rem;
    font-family: inherit;
    background: var(--bg-secondary);
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    color: var(--text-primary);
    transition: all var(--transition-normal);
}

.task-input:focus {
    outline: none;
    border-color: var(--accent-color);
    box-shadow: 0 0 0 4px rgba(102, 126, 234, 0.15);
}

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

.priority-select {
    padding: 16px 20px;
    font-size: 1rem;
    font-family: inherit;
    background: var(--bg-secondary);
    border: 2px solid var(--border-color);
    border-radius: var(--radius-md);
    color: var(--text-primary);
    cursor: pointer;
    transition: all var(--transition-normal);
    min-width: 140px;
}

.priority-select:focus {
    outline: none;
    border-color: var(--accent-color);
}

.add-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 16px 28px;
    font-size: 1rem;
    font-weight: 600;
    font-family: inherit;
    background: var(--accent-gradient);
    color: white;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all var(--transition-normal);
    box-shadow: 0 4px 15px rgba(102, 126, 234, 0.4);
}

.add-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(102, 126, 234, 0.5);
}

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

.add-icon {
    font-size: 1.25rem;
    font-weight: 700;
}

/* ========================================
   Controls Section
======================================== */
.controls-section {
    display: flex;
    flex-wrap: wrap;
    gap: 20px;
    padding: 20px 24px;
    background: var(--bg-glass);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-radius: var(--radius-xl);
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-lg);
}

.filter-group, .sort-group {
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.control-label {
    font-size: 0.85rem;
    font-weight: 600;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.filter-buttons {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
}

.filter-btn, .priority-filter-btn {
    padding: 10px 16px;
    font-size: 0.875rem;
    font-weight: 500;
    font-family: inherit;
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    color: var(--text-secondary);
    cursor: pointer;
    transition: all var(--transition-fast);
}

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

.filter-btn.active, .priority-filter-btn.active {
    background: var(--accent-gradient);
    color: white;
    border-color: transparent;
    box-shadow: 0 2px 10px rgba(102, 126, 234, 0.3);
}

.sort-select {
    padding: 10px 16px;
    font-size: 0.875rem;
    font-family: inherit;
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: var(--radius-sm);
    color: var(--text-primary);
    cursor: pointer;
    min-width: 200px;
}

/* ========================================
   Stats Section
======================================== */
.stats-section {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 16px;
}

.stat-card {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 4px;
    padding: 20px;
    background: var(--bg-glass);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-radius: var(--radius-lg);
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-lg);
}

.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-label {
    font-size: 0.875rem;
    color: var(--text-muted);
    font-weight: 500;
}

/* ========================================
   Tasks Section
======================================== */
.tasks-section {
    background: var(--bg-glass);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-radius: var(--radius-xl);
    border: 1px solid var(--border-color);
    padding: 24px;
    box-shadow: var(--shadow-lg);
}

.tasks-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
    flex-wrap: wrap;
    gap: 12px;
}

.tasks-header h2 {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
}

.clear-completed-btn {
    padding: 10px 18px;
    font-size: 0.875rem;
    font-weight: 500;
    font-family: inherit;
    background: var(--danger-color);
    color: white;
    border: none;
    border-radius: var(--radius-sm);
    cursor: pointer;
    transition: all var(--transition-normal);
    opacity: 0.9;
}

.clear-completed-btn:hover {
    opacity: 1;
    transform: translateY(-1px);
}

.tasks-list {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

/* ========================================
   Task Item
======================================== */
.task-item {
    display: flex;
    align-items: center;
    gap: 16px;
    padding: 18px 20px;
    background: var(--bg-secondary);
    border-radius: var(--radius-md);
    border: 1px solid var(--border-color);
    transition: all var(--transition-normal);
    animation: slideIn 0.3s ease;
}

@keyframes slideIn {
    from {
        opacity: 0;
        transform: translateY(-10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.task-item:hover {
    transform: translateX(4px);
    box-shadow: 0 4px 15px var(--shadow-color);
}

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

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

/* Priority Indicators */
.task-item.priority-high {
    border-left: 4px solid var(--priority-high);
}

.task-item.priority-medium {
    border-left: 4px solid var(--priority-medium);
}

.task-item.priority-low {
    border-left: 4px solid var(--priority-low);
}

/* Checkbox */
.task-checkbox {
    width: 24px;
    height: 24px;
    appearance: none;
    -webkit-appearance: none;
    background: var(--bg-tertiary);
    border: 2px solid var(--border-color);
    border-radius: 50%;
    cursor: pointer;
    position: relative;
    transition: all var(--transition-fast);
    flex-shrink: 0;
}

.task-checkbox:checked {
    background: var(--accent-gradient);
    border-color: transparent;
}

.task-checkbox:checked::after {
    content: '✓';
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    color: white;
    font-size: 14px;
    font-weight: 700;
}

.task-checkbox:hover {
    border-color: var(--accent-color);
}

/* Task Content */
.task-content {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 4px;
    min-width: 0;
}

.task-text {
    font-size: 1rem;
    font-weight: 500;
    color: var(--text-primary);
    word-break: break-word;
}

.task-meta {
    display: flex;
    gap: 12px;
    align-items: center;
    flex-wrap: wrap;
}

.task-priority {
    font-size: 0.75rem;
    font-weight: 600;
    padding: 4px 10px;
    border-radius: 20px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

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

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

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

.task-date {
    font-size: 0.75rem;
    color: var(--text-muted);
}

/* Delete Button */
.task-delete {
    width: 36px;
    height: 36px;
    display: flex;
    align-items: center;
    justify-content: center;
    background: transparent;
    border: none;
    border-radius: var(--radius-sm);
    color: var(--text-muted);
    cursor: pointer;
    font-size: 1.25rem;
    transition: all var(--transition-fast);
    flex-shrink: 0;
}

.task-delete:hover {
    background: var(--priority-high-bg);
    color: var(--danger-color);
}

/* ========================================
   Empty State
======================================== */
.empty-state {
    display: none;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: 60px 20px;
    text-align: center;
}

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

.empty-icon {
    font-size: 4rem;
    margin-bottom: 16px;
    opacity: 0.5;
}

.empty-title {
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 8px;
}

.empty-subtitle {
    font-size: 0.95rem;
    color: var(--text-muted);
}

/* ========================================
   Footer
======================================== */
.footer {
    margin-top: auto;
    padding: 24px;
    text-align: center;
}

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

/* ========================================
   Responsive Design
======================================== */

/* Tablet */
@media (max-width: 768px) {
    .app-container {
        padding: 16px;
    }
    
    .header-content {
        padding: 16px 20px;
    }
    
    .logo h1 {
        font-size: 1.5rem;
    }
    
    .logo-icon {
        width: 40px;
        height: 40px;
        font-size: 20px;
    }
    
    .input-group {
        flex-direction: column;
    }
    
    .task-input {
        min-width: 100%;
    }
    
    .priority-select {
        width: 100%;
    }
    
    .add-btn {
        width: 100%;
        justify-content: center;
    }
    
    .controls-section {
        flex-direction: column;
    }
    
    .filter-buttons {
        width: 100%;
    }
    
    .filter-btn, .priority-filter-btn {
        flex: 1;
        text-align: center;
    }
    
    .sort-select {
        width: 100%;
    }
    
    .stats-section {
        grid-template-columns: repeat(3, 1fr);
        gap: 12px;
    }
    
    .stat-card {
        padding: 16px;
    }
    
    .stat-number {
        font-size: 1.5rem;
    }
}

/* Mobile */
@media (max-width: 480px) {
    .app-container {
        padding: 12px;
    }
    
    .header-content {
        padding: 14px 16px;
        border-radius: var(--radius-lg);
    }
    
    .logo h1 {
        font-size: 1.25rem;
    }
    
    .logo-icon {
        width: 36px;
        height: 36px;
        font-size: 18px;
    }
    
    .theme-toggle {
        width: 44px;
        height: 44px;
    }
    
    .add-task-section, .controls-section, .tasks-section {
        padding: 16px;
        border-radius: var(--radius-lg);
    }
    
    .task-input, .priority-select, .add-btn {
        padding: 14px 16px;
    }
    
    .filter-btn, .priority-filter-btn {
        padding: 8px 12px;
        font-size: 0.8rem;
    }
    
    .stat-card {
        padding: 14px;
    }
    
    .stat-number {
        font-size: 1.25rem;
    }
    
    .stat-label {
        font-size: 0.75rem;
    }
    
    .task-item {
        padding: 14px 16px;
        gap: 12px;
    }
    
    .task-checkbox {
        width: 22px;
        height: 22px;
    }
    
    .task-text {
        font-size: 0.95rem;
    }
    
    .tasks-header h2 {
        font-size: 1.1rem;
    }
    
    .clear-completed-btn {
        padding: 8px 14px;
        font-size: 0.8rem;
    }
}

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

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

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

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

/* ========================================
   Selection Styling
======================================== */
::selection {
    background: var(--accent-color);
    color: white;
}

/* ========================================
   Focus Visible
======================================== */
:focus-visible {
    outline: 2px solid var(--accent-color);
    outline-offset: 2px;
}