:root {
    /* Light Mode */
    --bg: #F8FAFC;
    --surface: #FFFFFF;
    --surface-hover: #F1F5F9;
    --border: #E2E8F0;
    
    --primary: #3B82F6;
    --primary-hover: #2563EB;
    
    --text-main: #0F172A;
    --text-muted: #64748B;
    
    --radius-lg: 16px;
    --radius-md: 10px;
    --radius-sm: 6px;
    
    --transition: 200ms ease;
}

[data-theme="dark"] {
    /* Dark Mode */
    --bg: #0B0F19;
    --surface: #151A2A;
    --surface-hover: #1E253A;
    --border: #2A314A;
    
    --primary: #60A5FA;
    --primary-hover: #3B82F6;
    
    --text-main: #F1F5F9;
    --text-muted: #94A3B8;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

body {
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
    background-color: var(--bg);
    color: var(--text-main);
    line-height: 1.5;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    transition: background-color var(--transition), color var(--transition);
}

.visually-hidden {
    position: absolute;
    width: 1px;
    height: 1px;
    margin: -1px;
    border: 0;
    padding: 0;
    white-space: nowrap;
    clip-path: inset(100%);
    clip: rect(0 0 0 0);
    overflow: hidden;
}

.mt-4 { margin-top: 1rem; }

/* Header */
.app-header {
    height: 70px;
    background: rgba(var(--surface-rgb), 0.8);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    border-bottom: 1px solid var(--border);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 2rem;
    position: sticky;
    top: 0;
    z-index: 100;
}

[data-theme="dark"] .app-header { background: rgba(21, 26, 42, 0.8); }
:root:not([data-theme="dark"]) .app-header { background: rgba(255, 255, 255, 0.8); }

.logo {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    color: var(--primary);
}

.logo h1 {
    font-size: 1.25rem;
    font-weight: 700;
    color: var(--text-main);
    letter-spacing: -0.02em;
}

.logo span {
    font-weight: 400;
    color: var(--text-muted);
}

.theme-toggle .btn-icon {
    background: transparent;
    border: 1px solid var(--border);
    color: var(--text-muted);
    cursor: pointer;
    padding: 0.5rem;
    border-radius: var(--radius-sm);
    transition: var(--transition);
    display: flex;
    align-items: center;
    justify-content: center;
}

.theme-toggle .btn-icon:hover {
    color: var(--primary);
    background: var(--surface-hover);
}

/* Layout */
.app-layout {
    flex: 1;
    display: flex;
    justify-content: center;
    padding: 2rem;
}

.app-container {
    width: 100%;
    max-width: 900px;
}

/* Tabs */
.tabs {
    display: flex;
    gap: 0.5rem;
    background: var(--surface);
    padding: 0.5rem;
    border-radius: var(--radius-lg);
    border: 1px solid var(--border);
    margin-bottom: 2rem;
}

.tab-btn {
    flex: 1;
    background: transparent;
    border: none;
    padding: 0.75rem 1rem;
    border-radius: var(--radius-md);
    font-family: inherit;
    font-size: 1rem;
    font-weight: 600;
    color: var(--text-muted);
    cursor: pointer;
    transition: var(--transition);
}

.tab-btn:hover {
    color: var(--text-main);
}

.tab-btn.active {
    background: var(--surface-hover);
    color: var(--text-main);
    box-shadow: 0 2px 8px rgba(0,0,0,0.05);
}

.tab-content {
    display: none;
    animation: fadeIn 0.3s ease;
}

.tab-content.active {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(5px); }
    to { opacity: 1; transform: translateY(0); }
}

/* Encode Area */
.drag-drop-zone {
    border: 2px dashed var(--border);
    border-radius: var(--radius-lg);
    padding: 4rem 2rem;
    text-align: center;
    background: var(--surface);
    cursor: pointer;
    transition: var(--transition);
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 1rem;
}

.drag-drop-zone:hover, .drag-drop-zone.dragover {
    border-color: var(--primary);
    background: var(--surface-hover);
}

.drag-drop-zone svg {
    color: var(--primary);
    margin-bottom: 0.5rem;
}

.drag-drop-zone h3 {
    font-size: 1.25rem;
    font-weight: 600;
}

.drag-drop-zone p {
    color: var(--text-muted);
}

.file-limits {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-top: 1rem;
}

/* Results Area */
.result-area {
    margin-top: 2rem;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-lg);
    padding: 2rem;
}

.file-meta {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
    gap: 1.5rem;
    margin-bottom: 2rem;
    padding-bottom: 1.5rem;
    border-bottom: 1px solid var(--border);
}

.meta-item {
    display: flex;
    flex-direction: column;
    gap: 0.25rem;
}

.meta-label {
    font-size: 0.8rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.05em;
    font-weight: 600;
}

.meta-value {
    font-weight: 500;
    font-size: 0.95rem;
    word-break: break-all;
}

/* Outputs */
.preview-wrap {
    margin-bottom: 2rem;
}

.preview-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 0.75rem;
}

.preview-header span {
    font-weight: 600;
    font-size: 0.95rem;
}

.btn-group {
    display: flex;
    gap: 0.5rem;
}

.code-editor {
    width: 100%;
    height: 140px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    padding: 1rem;
    color: var(--text-main);
    font-family: 'Fira Code', monospace;
    font-size: 0.85rem;
    line-height: 1.5;
    resize: vertical;
    outline: none;
    transition: var(--transition);
}

.code-editor:focus {
    border-color: var(--primary);
}

.code-editor[readonly] {
    cursor: default;
}

.preview-visual {
    width: 100%;
    min-height: 200px;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius-md);
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
    padding: 1rem;
}

.preview-visual img {
    max-width: 100%;
    max-height: 400px;
    object-fit: contain;
    border-radius: var(--radius-sm);
}

/* Decode Tab Inputs */
.input-wrap label {
    display: block;
    margin-bottom: 0.75rem;
    font-weight: 600;
}

/* Buttons */
.btn {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.6rem 1.2rem;
    border-radius: var(--radius-md);
    font-family: inherit;
    font-size: 0.95rem;
    font-weight: 500;
    cursor: pointer;
    transition: var(--transition);
    border: 1px solid transparent;
}
.btn-sm {
    padding: 0.4rem 0.8rem;
    font-size: 0.85rem;
}
.btn-primary {
    background: var(--primary);
    color: #fff;
}
.btn-primary:hover { background: var(--primary-hover); }

.btn-outline {
    background: transparent;
    color: var(--text-main);
    border-color: var(--border);
}
.btn-outline:hover { background: var(--surface-hover); }

/* Toast */
.toast {
    position: fixed;
    bottom: -50px;
    left: 50%;
    transform: translateX(-50%);
    background: var(--text-main);
    color: var(--bg);
    padding: 0.75rem 1.5rem;
    border-radius: 30px;
    font-size: 0.9rem;
    font-weight: 600;
    transition: bottom 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
    z-index: 1000;
    box-shadow: 0 10px 25px rgba(0,0,0,0.1);
}
.toast.show {
    bottom: 2rem;
}

/* Responsive */
@media (max-width: 640px) {
    .app-header { padding: 0 1rem; }
    .app-layout { padding: 1rem; }
    .tabs { flex-direction: column; }
    .file-meta { grid-template-columns: 1fr 1fr; }
}