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/layout/ThemeToggle.tsx

@@ -0,0 +1,38 @@
+'use client';
+
+import * as React from 'react';
+import { Moon, Sun } from 'lucide-react';
+import { Button } from '@/components/ui/Button';
+
+export function ThemeToggle() {
+  const [theme, setTheme] = React.useState<'light' | 'dark'>('light');
+
+  React.useEffect(() => {
+    // Get theme from localStorage or system preference
+    const savedTheme = localStorage.getItem('theme') as 'light' | 'dark' | null;
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    const initialTheme = savedTheme || (prefersDark ? 'dark' : 'light');
+
+    setTheme(initialTheme);
+    document.documentElement.classList.toggle('dark', initialTheme === 'dark');
+  }, []);
+
+  const toggleTheme = () => {
+    const newTheme = theme === 'light' ? 'dark' : 'light';
+    setTheme(newTheme);
+    localStorage.setItem('theme', newTheme);
+    document.documentElement.classList.toggle('dark', newTheme === 'dark');
+  };
+
+  return (
+    <Button
+      variant="ghost"
+      size="icon"
+      onClick={toggleTheme}
+      aria-label="Toggle theme"
+      className="rounded-full"
+    >
+      {theme === 'light' ? <Moon className="h-5 w-5" /> : <Sun className="h-5 w-5" />}
+    </Button>
+  );
+}