feat: initialize Convert UI - browser-based file conversion app

- Add Next.js 16 with Turbopack and React 19
- Add Tailwind CSS 4 with OKLCH color system
- Implement FFmpeg.wasm for video/audio conversion
- Implement ImageMagick WASM for image conversion
- Add file upload with drag-and-drop
- Add format selector with fuzzy search
- Add conversion preview and download
- Add conversion history with localStorage
- Add dark/light theme support
- Support 22+ file formats across video, audio, and images

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/ui/Card.tsx

@@ -0,0 +1,57 @@
+import * as React from 'react';
+import { cn } from '@/lib/utils/cn';
+
+const Card = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div
+      ref={ref}
+      className={cn('rounded-lg border bg-card text-card-foreground shadow-sm', className)}
+      {...props}
+    />
+  )
+);
+Card.displayName = 'Card';
+
+const CardHeader = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div ref={ref} className={cn('flex flex-col space-y-1.5 p-6', className)} {...props} />
+  )
+);
+CardHeader.displayName = 'CardHeader';
+
+const CardTitle = React.forwardRef<HTMLParagraphElement, React.HTMLAttributes<HTMLHeadingElement>>(
+  ({ className, children, ...props }, ref) => (
+    <h3
+      ref={ref}
+      className={cn('text-2xl font-semibold leading-none tracking-tight', className)}
+      {...props}
+    >
+      {children}
+    </h3>
+  )
+);
+CardTitle.displayName = 'CardTitle';
+
+const CardDescription = React.forwardRef<
+  HTMLParagraphElement,
+  React.HTMLAttributes<HTMLParagraphElement>
+>(({ className, ...props }, ref) => (
+  <p ref={ref} className={cn('text-sm text-muted-foreground', className)} {...props} />
+));
+CardDescription.displayName = 'CardDescription';
+
+const CardContent = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div ref={ref} className={cn('p-6 pt-0', className)} {...props} />
+  )
+);
+CardContent.displayName = 'CardContent';
+
+const CardFooter = React.forwardRef<HTMLDivElement, React.HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div ref={ref} className={cn('flex items-center p-6 pt-0', className)} {...props} />
+  )
+);
+CardFooter.displayName = 'CardFooter';
+
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent };