feat(01-04): create Google Sign-In button component

- Add cn() utility function for class name merging
- Add Button component with default/outline/ghost variants
- Add Card component with Header, Title, Description, Content, Footer
- Add GoogleSignInButton with signInWithOAuth for Google provider
- Italian text: 'Accedi con Google'
- Redirects to /auth/callback after consent

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/utils.ts

@@ -0,0 +1,6 @@
+import { type ClassValue, clsx } from 'clsx'
+import { twMerge } from 'tailwind-merge'
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs))
+}