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/Button.tsx

@@ -0,0 +1,45 @@
+import * as React from 'react';
+import { cn } from '@/lib/utils/cn';
+
+export interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: 'default' | 'destructive' | 'outline' | 'secondary' | 'ghost' | 'link';
+  size?: 'default' | 'sm' | 'lg' | 'icon';
+  children?: React.ReactNode;
+}
+
+const Button = React.forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ className, variant = 'default', size = 'default', ...props }, ref) => {
+    return (
+      <button
+        className={cn(
+          'inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium ring-offset-background transition-colors',
+          'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2',
+          'disabled:pointer-events-none disabled:opacity-50',
+          {
+            'bg-primary text-primary-foreground hover:bg-primary/90': variant === 'default',
+            'bg-destructive text-destructive-foreground hover:bg-destructive/90':
+              variant === 'destructive',
+            'border border-input bg-background hover:bg-accent hover:text-accent-foreground':
+              variant === 'outline',
+            'bg-secondary text-secondary-foreground hover:bg-secondary/80': variant === 'secondary',
+            'hover:bg-accent hover:text-accent-foreground': variant === 'ghost',
+            'text-primary underline-offset-4 hover:underline': variant === 'link',
+          },
+          {
+            'h-10 px-4 py-2': size === 'default',
+            'h-9 rounded-md px-3': size === 'sm',
+            'h-11 rounded-md px-8': size === 'lg',
+            'h-10 w-10': size === 'icon',
+          },
+          className
+        )}
+        ref={ref}
+        {...props}
+      />
+    );
+  }
+);
+
+Button.displayName = 'Button';
+
+export { Button };