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/globals.css

@@ -1,4 +1,5 @@
 @import "tailwindcss";
+@plugin "tailwind-scrollbar";
 
 @source "../components/*.{js,ts,jsx,tsx}";
 @source "../components/ui/*.{js,ts,jsx,tsx}";
@@ -85,14 +86,13 @@
   }
 }
 
-:root, .dark {
-  color-scheme: dark;
+:root {
   /* CORPORATE DARK THEME (The Standard) */
   --background: #0a0a0f;
   --foreground: #ffffff;
   --card: rgba(255, 255, 255, 0.03);
   --card-foreground: #ffffff;
-  --popover: #0f0f15;
+  --popover: #5D429C;
   --popover-foreground: #ffffff;
   --primary: #8b5cf6;
   --primary-foreground: #ffffff;
@@ -110,36 +110,12 @@
   --radius: 1rem;
 }
 
-.light {
-  color-scheme: light;
-  /* LIGHT ADAPTATION (Keeping the "Glass" look) */
-  --background: oklch(98% 0.005 255);
-  --foreground: oklch(20% 0.04 255);
-  --card: rgba(255, 255, 255, 0.4);
-  --card-foreground: oklch(20% 0.04 255);
-  --popover: oklch(100% 0 0);
-  --popover-foreground: oklch(20% 0.04 255);
-  --primary: oklch(55% 0.22 270);
-  --primary-foreground: oklch(100% 0 0);
-  --secondary: rgba(0, 0, 0, 0.02);
-  --secondary-foreground: oklch(20% 0.04 255);
-  --muted: rgba(0, 0, 0, 0.02);
-  --muted-foreground: oklch(45% 0.04 255);
-  --accent: rgba(0, 0, 0, 0.03);
-  --accent-foreground: oklch(15% 0.05 255);
-  --destructive: oklch(60% 0.2 25);
-  --destructive-foreground: oklch(100% 0 0);
-  --border: rgba(0, 0, 0, 0.2);
-  --input: rgba(0, 0, 0, 0.08);
-  --ring: rgba(139, 92, 246, 0.4);
-}
-
 @layer base {
   * {
     @apply border-border outline-ring/50;
   }
   body {
-    @apply bg-background text-foreground;
+    @apply bg-background text-foreground scrollbar-thin scrollbar-thumb-primary/20 scrollbar-track-transparent hover:scrollbar-thumb-primary/40;
     font-family: var(--font-sans);
     -webkit-font-smoothing: antialiased;
     -moz-osx-font-smoothing: grayscale;