feat: refactor theme, add tailwind-scrollbar, and improve UI components

- Removed manual theme switching logic and ThemeProvider
- Installed and configured tailwind-scrollbar plugin
- Updated FileConverter and ConversionOptions to use shadcn Input
- Refactored FontSelector to use shadcn Tabs
- Simplified global styles and adjusted glassmorphic effects

app/not-found.tsx

@@ -1,6 +1,5 @@
 'use client';
 
-import * as React from 'react';
 import Link from 'next/link';
 import { motion } from 'framer-motion';
 import AnimatedBackground from '@/components/AnimatedBackground';
@@ -10,11 +9,6 @@ import { Button } from '@/components/ui/button';
 import { Home } from 'lucide-react';
 
 export default function NotFound() {
-  React.useEffect(() => {
-    // Force dark mode on html element for the 404 page
-    document.documentElement.classList.remove('light');
-    document.documentElement.classList.add('dark');
-  }, []);
 
   return (
     <main className="relative min-h-screen dark text-foreground flex flex-col">