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

components/units/SearchUnits.tsx

@@ -134,7 +134,7 @@ export default function SearchUnits({ onSelectUnit, className }: SearchUnitsProp
 
       {/* Results dropdown */}
       {isOpen && results.length > 0 && (
-        <div className="absolute z-50 w-full mt-2 bg-popover border rounded-lg shadow-lg max-h-80 overflow-y-auto">
+        <div className="absolute z-50 w-full mt-2 bg-popover border rounded-lg shadow-lg max-h-80 overflow-y-auto scrollbar">
           {results.map((result, index) => (
             <button
               key={`${result.measure}-${result.unitInfo.abbr}`}