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/components/ui/card.tsx

@@ -0,0 +1,69 @@
+import { HTMLAttributes, forwardRef } from 'react'
+import { cn } from '@/lib/utils'
+
+const Card = forwardRef<HTMLDivElement, HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div
+      ref={ref}
+      className={cn(
+        'rounded-lg border border-gray-200 bg-white shadow-sm',
+        className
+      )}
+      {...props}
+    />
+  )
+)
+Card.displayName = 'Card'
+
+const CardHeader = forwardRef<HTMLDivElement, HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div
+      ref={ref}
+      className={cn('flex flex-col space-y-1.5 p-6', className)}
+      {...props}
+    />
+  )
+)
+CardHeader.displayName = 'CardHeader'
+
+const CardTitle = forwardRef<HTMLParagraphElement, HTMLAttributes<HTMLHeadingElement>>(
+  ({ className, ...props }, ref) => (
+    <h3
+      ref={ref}
+      className={cn('text-2xl font-semibold leading-none tracking-tight', className)}
+      {...props}
+    />
+  )
+)
+CardTitle.displayName = 'CardTitle'
+
+const CardDescription = forwardRef<HTMLParagraphElement, HTMLAttributes<HTMLParagraphElement>>(
+  ({ className, ...props }, ref) => (
+    <p
+      ref={ref}
+      className={cn('text-sm text-gray-500', className)}
+      {...props}
+    />
+  )
+)
+CardDescription.displayName = 'CardDescription'
+
+const CardContent = forwardRef<HTMLDivElement, HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div ref={ref} className={cn('p-6 pt-0', className)} {...props} />
+  )
+)
+CardContent.displayName = 'CardContent'
+
+const CardFooter = forwardRef<HTMLDivElement, HTMLAttributes<HTMLDivElement>>(
+  ({ className, ...props }, ref) => (
+    <div
+      ref={ref}
+      className={cn('flex items-center p-6 pt-0', className)}
+      {...props}
+    />
+  )
+)
+CardFooter.displayName = 'CardFooter'
+
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent }