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

components/BackToTop.tsx

@@ -44,7 +44,7 @@ export default function BackToTop() {
       {isVisible && (
         <motion.button
           onClick={scrollToTop}
-          className="fixed bottom-8 right-8 p-4 rounded-full glass hover:bg-white/10 text-purple-400 hover:text-purple-300 transition-colors shadow-lg z-40 group"
+          className="fixed bottom-8 right-8 p-4 rounded-full glass hover:bg-accent/50 text-purple-400 hover:text-purple-300 transition-colors shadow-lg z-40 group"
           initial={{ opacity: 0, y: 20 }}
           animate={{ opacity: 1, y: 0 }}
           exit={{ opacity: 0, y: 20 }}