/* ================================
   CSS Custom Properties (Variables)
   ================================ */
:root {
    /* Colors - Light Theme */
    --color-primary: #667eea;
    --color-primary-dark: #5a67d8;
    --color-primary-light: #a3bffa;
    
    --color-success: #48bb78;
    --color-warning: #ed8936;
    --color-danger: #f56565;
    
    --color-bg-primary: #f7fafc;
    --color-bg-secondary: #ffffff;
    --color-bg-tertiary: #edf2f7;
    
    --color-text-primary: #1a202c;
    --color-text-secondary: #4a5568;
    --color-text-tertiary: #718096;
    --color-text-inverse: #ffffff;
    
    --color-border: #e2e8f0;
    --color-shadow: rgba(0, 0, 0, 0.1);
    
    /* Typography */
    --font-family-base: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 
                        Oxygen, Ubuntu, Cantarell, 'Fira Sans', 'Droid Sans', 
                        'Helvetica Neue', sans-serif;
    --font-family-mono: 'SF Mono', 'Fira Code', 'Fira Mono', Menlo, Monaco, 
                        Consolas, 'Liberation Mono', 'Courier New', monospace;
    
    --font-size-xs: 0.75rem;    /* 12px */
    --font-size-sm: 0.875rem;   /* 14px */
    --font-size-base: 1rem;     /* 16px */
    --font-size-lg: 1.125rem;   /* 18px */
    --font-size-xl: 1.25rem;    /* 20px */
    --font-size-2xl: 1.5rem;    /* 24px */
    --font-size-3xl: 1.875rem;  /* 30px */
    
    --font-weight-normal: 400;
    --font-weight-medium: 500;
    --font-weight-semibold: 600;
    --font-weight-bold: 700;
    
    --line-height-tight: 1.25;
    --line-height-normal: 1.5;
    --line-height-relaxed: 1.75;
    
    /* Spacing */
    --spacing-xs: 0.25rem;   /* 4px */
    --spacing-sm: 0.5rem;    /* 8px */
    --spacing-md: 1rem;      /* 16px */
    --spacing-lg: 1.5rem;    /* 24px */
    --spacing-xl: 2rem;      /* 32px */
    --spacing-2xl: 3rem;     /* 48px */
    
    /* Border Radius */
    --radius-sm: 0.25rem;
    --radius-md: 0.5rem;
    --radius-lg: 0.75rem;
    --radius-xl: 1rem;
    --radius-full: 9999px;
    
    /* Shadows */
    --shadow-sm: 0 1px 2px 0 var(--color-shadow);
    --shadow-md: 0 4px 6px -1px var(--color-shadow);
    --shadow-lg: 0 10px 15px -3px var(--color-shadow);
    --shadow-xl: 0 20px 25px -5px var(--color-shadow);
    
    /* Transitions */
    --transition-fast: 150ms ease;
    --transition-normal: 250ms ease;
    --transition-slow: 350ms ease;
    
    /* Z-Index Scale */
    --z-dropdown: 100;
    --z-sticky: 200;
    --z-fixed: 300;
    --z-modal: 400;
    --z-toast: 500;
    
    /* Container */
    --container-max-width: 42rem;
    --container-padding: var(--spacing-md);
    
    /* Safe Area Insets (for notched devices) */
    --safe-area-inset-top: env(safe-area-inset-top, 0px);
    --safe-area-inset-right: env(safe-area-inset-right, 0px);
    --safe-area-inset-bottom: env(safe-area-inset-bottom, 0px);
    --safe-area-inset-left: env(safe-area-inset-left, 0px);
}

/* Dark Theme */
[data-theme="dark"] {
    --color-bg-primary: #1a202c;
    --color-bg-secondary: #2d3748;
    --color-bg-tertiary: #4a5568;
    
    --color-text-primary: #f7fafc;
    --color-text-secondary: #e2e8f0;
    --color-text-tertiary: #a0aec0;
    
    --color-border: #4a5568;
    --color-shadow: rgba(0, 0, 0, 0.3);
}

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

html {
    font-size: 16px;
    -webkit-text-size-adjust: 100%;
    -moz-text-size-adjust: 100%;
    text-size-adjust: 100%;
    scroll-behavior: smooth;
}

body {
    font-family: var(--font-family-base);
    font-size: var(--font-size-base);
    font-weight: var(--font-weight-normal);
    line-height: var(--line-height-normal);
    color: var(--color-text-primary);
    background-color: var(--color-bg-primary);
    min-height: 100vh;
    min-height: 100dvh;
    overflow-x: hidden;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

/* Focus Visible - Accessibility */
:focus-visible {
    outline: 3px solid var(--color-primary);
    outline-offset: 2px;
}

:focus:not(:focus-visible) {
    outline: none;
}

/* Selection */
::selection {
    background-color: var(--color-primary);
    color: var(--color-text-inverse);
}

/* ================================
   Utility Classes
   ================================ */
.visually-hidden {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border: 0;
}

.hidden {
    display: none !important;
}

/* ================================
   App Container
   ================================ */
.app-container {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
    min-height: 100dvh;
}

/* ================================
   Header
   ================================ */
.header {
    position: sticky;
    top: 0;
    z-index: var(--z-sticky);
    background: linear-gradient(135deg, var(--color-primary), var(--color-primary-dark));
    color: var(--color-text-inverse);
    padding: var(--spacing-md);
    padding-top: calc(var(--spacing-md) + var(--safe-area-inset-top));
    box-shadow: var(--shadow-md);
}

.header__content {
    display: flex;
    justify-content: space-between;
    align-items: center;
    max-width: var(--container-max-width);
    margin: 0 auto;
}

.header__title {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-size: var(--font-size-xl);
    font-weight: var(--font-weight-bold);
}

.header__logo {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 2rem;
    height: 2rem;
    background-color: rgba(255, 255, 255, 0.2);
    border-radius: var(--radius-md);
    font-size: var(--font-size-lg);
}

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

/* ================================
   Main Content
   ================================ */
.main {
    flex: 1;
    width: 100%;
    max-width: var(--container-max-width);
    margin: 0 auto;
    padding: var(--spacing-md);
    padding-bottom: calc(var(--spacing-md) + var(--safe-area-inset-bottom));
}

/* ================================
   Sections
   ================================ */
.section {
    margin-bottom: var(--spacing-lg);
}

/* ================================
   Status Banner
   ================================ */
.status-banner {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: var(--spacing-sm) var(--spacing-md);
    background-color: var(--color-primary-light);
    color: var(--color-primary-dark);
    border-radius: var(--radius-md);
    margin-bottom: var(--spacing-md);
    animation: slideDown var(--transition-normal);
}

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

.status-banner__close {
    background: none;
    border: none;
    font-size: var(--font-size-xl);
    cursor: pointer;
    color: inherit;
    line-height: 1;
    padding: var(--spacing-xs);
}

/* ================================
   Forms
   ================================ */
.task-form {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-md);
    padding: var(--spacing-lg);
    background-color: var(--color-bg-secondary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-md);
}

.form-group {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-xs);
}

.form-label {
    font-size: var(--font-size-sm);
    font-weight: var(--font-weight-medium);
    color: var(--color-text-secondary);
}

.input-wrapper {
    position: relative;
    display: flex;
    align-items: center;
}

.form-input {
    width: 100%;
    padding: var(--spacing-sm) var(--spacing-md);
    padding-right: 4rem;
    font-size: var(--font-size-base);
    font-family: inherit;
    color: var(--color-text-primary);
    background-color: var(--color-bg-tertiary);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.form-input:hover {
    border-color: var(--color-primary-light);
}

.form-input:focus {
    border-color: var(--color-primary);
    box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.2);
    outline: none;
}

.form-input::placeholder {
    color: var(--color-text-tertiary);
}

.form-input:invalid:not(:placeholder-shown) {
    border-color: var(--color-danger);
}

.input-counter {
    position: absolute;
    right: var(--spacing-sm);
    font-size: var(--font-size-xs);
    color: var(--color-text-tertiary);
}

.form-select {
    width: 100%;
    padding: var(--spacing-sm) var(--spacing-md);
    font-size: var(--font-size-base);
    font-family: inherit;
    color: var(--color-text-primary);
    background-color: var(--color-bg-tertiary);
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    cursor: pointer;
    appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 12 12'%3E%3Cpath fill='%234a5568' d='M6 8L1 3h10z'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right var(--spacing-md) center;
}

.form-help {
    font-size: var(--font-size-xs);
    color: var(--color-text-tertiary);
}

.form-error {
    font-size: var(--font-size-xs);
    color: var(--color-danger);
}

/* ================================
   Buttons
   ================================ */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: var(--spacing-xs);
    padding: var(--spacing-sm) var(--spacing-lg);
    font-size: var(--font-size-base);
    font-weight: var(--font-weight-medium);
    font-family: inherit;
    line-height: var(--line-height-tight);
    text-decoration: none;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all var(--transition-fast);
    user-select: none;
    -webkit-tap-highlight-color: transparent;
}

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

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

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

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

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

.btn--danger:hover:not(:disabled) {
    background-color: #e53e3e;
}

.btn--icon {
    padding: var(--spacing-sm);
    background-color: rgba(255, 255, 255, 0.2);
    border-radius: var(--radius-md);
    font-size: var(--font-size-lg);
}

.btn--icon:hover:not(:disabled) {
    background-color: rgba(255, 255, 255, 0.3);
}

.btn--sm {
    padding: var(--spacing-xs) var(--spacing-md);
    font-size: var(--font-size-sm);
}

/* ================================
   Filters
   ================================ */
.filters {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-sm);
    padding: var(--spacing-sm);
    background-color: var(--color-bg-secondary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
}

.filter-btn {
    flex: 1;
    min-width: max-content;
    padding: var(--spacing-sm) var(--spacing-md);
    font-size: var(--font-size-sm);
    font-weight: var(--font-weight-medium);
    font-family: inherit;
    color: var(--color-text-secondary);
    background-color: transparent;
    border: none;
    border-radius: var(--radius-md);
    cursor: pointer;
    transition: all var(--transition-fast);
}

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

.filter-btn--active {
    color: var(--color-text-inverse);
    background-color: var(--color-primary);
}

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

.badge {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    min-width: 1.5rem;
    height: 1.25rem;
    padding: 0 var(--spacing-xs);
    margin-left: var(--spacing-xs);
    font-size: var(--font-size-xs);
    font-weight: var(--font-weight-semibold);
    background-color: rgba(0, 0, 0, 0.1);
    border-radius: var(--radius-full);
}

.filter-btn--active .badge {
    background-color: rgba(255, 255, 255, 0.2);
}

/* ================================
   Task List
   ================================ */
.task-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.task-item {
    display: flex;
    align-items: flex-start;
    gap: var(--spacing-md);
    padding: var(--spacing-md);
    background-color: var(--color-bg-secondary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-sm);
    transition: all var(--transition-fast);
    animation: fadeIn var(--transition-normal);
}

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

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

.task-item--completed {
    opacity: 0.7;
}

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

.task-item--high {
    border-left: 4px solid var(--color-danger);
}

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

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

/* Custom Checkbox */
.task-item__checkbox {
    position: relative;
    flex-shrink: 0;
}

.task-item__checkbox input {
    position: absolute;
    opacity: 0;
    width: 1.5rem;
    height: 1.5rem;
    cursor: pointer;
}

.task-item__checkbox-custom {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 1.5rem;
    height: 1.5rem;
    border: 2px solid var(--color-border);
    border-radius: var(--radius-md);
    background-color: var(--color-bg-tertiary);
    transition: all var(--transition-fast);
}

.task-item__checkbox input:checked + .task-item__checkbox-custom {
    background-color: var(--color-success);
    border-color: var(--color-success);
}

.task-item__checkbox input:checked + .task-item__checkbox-custom::after {
    content: '✓';
    color: white;
    font-size: var(--font-size-sm);
    font-weight: var(--font-weight-bold);
}

.task-item__checkbox input:focus-visible + .task-item__checkbox-custom {
    box-shadow: 0 0 0 3px rgba(72, 187, 120, 0.3);
}

.task-item__content {
    flex: 1;
    min-width: 0;
}

.task-item__text {
    word-wrap: break-word;
    overflow-wrap: break-word;
    hyphens: auto;
}

.task-item__meta {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-sm);
    margin-top: var(--spacing-xs);
    font-size: var(--font-size-xs);
    color: var(--color-text-tertiary);
}

.task-item__actions {
    display: flex;
    gap: var(--spacing-xs);
    flex-shrink: 0;
}

.task-item__btn {
    padding: var(--spacing-xs);
    font-size: var(--font-size-base);
    background: none;
    border: none;
    border-radius: var(--radius-sm);
    cursor: pointer;
    opacity: 0.6;
    transition: opacity var(--transition-fast), background-color var(--transition-fast);
}

.task-item__btn:hover {
    opacity: 1;
    background-color: var(--color-bg-tertiary);
}

.task-item__btn--delete:hover {
    background-color: rgba(245, 101, 101, 0.1);
}

/* ================================
   Empty State
   ================================ */
.empty-state {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    padding: var(--spacing-2xl);
    text-align: center;
}

.empty-state__icon {
    font-size: 3rem;
    margin-bottom: var(--spacing-md);
    opacity: 0.5;
}

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

/* ================================
   Footer
   ================================ */
.footer {
    padding: var(--spacing-md);
    padding-bottom: calc(var(--spacing-md) + var(--safe-area-inset-bottom));
    text-align: center;
    background-color: var(--color-bg-secondary);
    border-top: 1px solid var(--color-border);
}

.footer__text {
    font-size: var(--font-size-sm);
    color: var(--color-text-tertiary);
}

.offline-indicator {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-xs);
    padding: var(--spacing-xs) var(--spacing-sm);
    margin-right: var(--spacing-sm);
    background-color: var(--color-warning);
    color: var(--color-text-inverse);
    border-radius: var(--radius-full);
    font-size: var(--font-size-xs);
    font-weight: var(--font-weight-medium);
}

/* ================================
   Toast Notifications
   ================================ */
.toast-container {
    position: fixed;
    bottom: calc(var(--spacing-md) + var(--safe-area-inset-bottom));
    left: 50%;
    transform: translateX(-50%);
    z-index: var(--z-toast);
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
    width: calc(100% - var(--spacing-xl));
    max-width: 24rem;
    pointer-events: none;
}

.toast {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    padding: var(--spacing-md);
    background-color: var(--color-bg-secondary);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-lg);
    pointer-events: auto;
    animation: toastIn var(--transition-normal);
}

@keyframes toastIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.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 {
    flex-shrink: 0;
    font-size: var(--font-size-xl);
}

.toast__message {
    flex: 1;
    font-size: var(--font-size-sm);
}

.toast__close {
    flex-shrink: 0;
    padding: var(--spacing-xs);
    background: none;
    border: none;
    cursor: pointer;
    opacity: 0.6;
    font-size: var(--font-size-lg);
    line-height: 1;
}

.toast__close:hover {
    opacity: 1;
}

/* ================================
   Responsive Breakpoints
   ================================ */

/* Small devices (landscape phones) */
@media (min-width: 576px) {
    .task-form {
        flex-direction: row;
        flex-wrap: wrap;
    }
    
    .task-form .form-group:first-child {
        flex: 1;
        min-width: 200px;
    }
    
    .task-form .form-group:nth-child(2) {
        width: auto;
    }
    
    .task-form .btn {
        align-self: flex-end;
    }
}

/* Medium devices (tablets) */
@media (min-width: 768px) {
    :root {
        --container-padding: var(--spacing-lg);
    }
    
    .header {
        padding: var(--spacing-lg);
    }
    
    .header__title {
        font-size: var(--font-size-2xl);
    }
    
    .task-form {
        padding: var(--spacing-xl);
    }
    
    .filters {
        justify-content: center;
    }
    
    .filter-btn {
        flex: 0 1 auto;
    }
}

/* Large devices (desktops) */
@media (min-width: 992px) {
    .header__content {
        max-width: var(--container-max-width);
    }
}

/* Reduced motion preference */
@media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        transition-duration: 0.01ms !important;
        scroll-behavior: auto !important;
    }
}

/* High contrast mode */
@media (prefers-contrast: high) {
    :root {
        --color-border: currentColor;
    }
    
    .btn {
        border: 2px solid currentColor;
    }
}

/* Print styles */
@media print {
    .header,
    .footer,
    .task-form,
    .filters,
    .task-item__actions {
        display: none !important;
    }
    
    .main {
        max-width: 100%;
    }
    
    .task-item {
        break-inside: avoid;
        box-shadow: none;
        border: 1px solid #000;
    }
}