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/(app)/layout.tsx

@@ -1,3 +1,4 @@
+import AnimatedBackground from '@/components/AnimatedBackground';
 import { AppShell } from '@/components/layout/AppShell';
 import { Providers } from '@/components/providers/Providers';