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/layout/AppShell.tsx

@@ -13,12 +13,12 @@ interface AppShellProps {
 export function AppShell({ children }: AppShellProps) {
   return (
     <SidebarProvider>
-      <div className="flex h-screen overflow-hidden bg-background text-foreground relative">
+      <div className="flex h-screen overflow-hidden bg-transparent text-foreground relative">
         <AnimatedBackground />
         <AppSidebar />
         <div className="flex-1 flex flex-col min-w-0 relative z-10">
           <AppHeader />
-          <main className="flex-1 overflow-y-auto">
+          <main className="flex-1 overflow-y-auto scrollbar">
             {children}
           </main>
         </div>