refactor: streamline toast system and harmonize UI across tools

- Migrate all toast notifications to sonner and remove custom ToastProvider
- Align Card and TextInput styling across Figlet and Pastel (rounded-lg, border-based)
- Fix build error by removing non-existent export in lib/units/index.ts
- Clean up unused Figlet components and constants

app/globals.css

@@ -110,6 +110,7 @@
 }
 
 :root, .dark {
+  color-scheme: dark;
   /* CORPORATE DARK THEME (The Standard) */
   --background: #0a0a0f;
   --foreground: #ffffff;
@@ -134,6 +135,7 @@
 }
 
 .light {
+  color-scheme: light;
   /* LIGHT ADAPTATION (Keeping the "Glass" look) */
   --background: oklch(98% 0.005 255);
   --foreground: oklch(20% 0.04 255);
@@ -166,6 +168,10 @@
     -webkit-font-smoothing: antialiased;
     -moz-osx-font-smoothing: grayscale;
   }
+  /* Fix native select dropdown styling */
+  select option {
+    @apply bg-popover text-popover-foreground;
+  }
 }
 
 html {