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/layout/AppHeader.tsx

@@ -17,7 +17,7 @@ export function AppHeader() {
   const pathSegments = pathname.split('/').filter(Boolean);
   
   return (
-    <header className="h-16 border-b border-white/5 bg-background/10 backdrop-blur-xl sticky top-0 z-40 flex items-center justify-between px-4 lg:px-8">
+    <header className="h-16 border-b border-border bg-background/10 backdrop-blur-xl sticky top-0 z-40 flex items-center justify-between px-4 lg:px-8">
       <div className="flex items-center gap-4">
         <Button 
           variant="ghost" 
@@ -69,7 +69,7 @@ function ThemeToggleComponent() {
       variant="ghost" 
       size="icon"
       onClick={() => setTheme(resolvedTheme === 'dark' ? 'light' : 'dark')}
-      className="text-muted-foreground hover:text-foreground hover:bg-white/5"
+      className="text-muted-foreground hover:text-foreground hover:bg-accent/50"
       title={`Switch to ${resolvedTheme === 'dark' ? 'light' : 'dark'} mode`}
     >
       {resolvedTheme === 'dark' ? (