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/(app)/units/page.tsx

@@ -2,12 +2,16 @@ import MainConverter from '@/components/units/converter/MainConverter';
 
 export default function UnitsPage() {
   return (
-    <div className="p-4 sm:p-8 max-w-7xl mx-auto">
-      <div className="mb-8">
-        <h1 className="text-3xl font-bold mb-2">Units Converter</h1>
-        <p className="text-muted-foreground italic">Smart unit converter with 187 units across 23 categories</p>
+    <div className="min-h-screen py-12">
+      <div className="max-w-7xl mx-auto px-8 space-y-8">
+        <div>
+          <h1 className="text-4xl font-bold mb-2">Units Converter</h1>
+          <p className="text-muted-foreground">
+            Smart unit converter with 187 units across 23 categories
+          </p>
+        </div>
+        <MainConverter />
       </div>
-      <MainConverter />
     </div>
   );
 }