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

package.json

@@ -46,6 +46,7 @@
     "eslint-config-next": "^15.1.7",
     "postcss": "^8.5.6",
     "shadcn": "^3.8.5",
+    "tailwind-scrollbar": "^4.0.2",
     "tailwindcss": "^4.2.0",
     "tw-animate-css": "^1.4.0",
     "typescript": "^5.9.3"