feat(01-04): add Google button to login and register pages

- Add auth layout with centered card design
- Add Input component for form fields
- Add LoginForm component with email/password and validation
- Add RegisterForm component with password requirements
- Add login page with Google button + 'oppure' divider + email form
- Add register page with Google button + 'oppure' divider + email form
- Italian text throughout (Accedi, Registrati, oppure)

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

src/components/ui/input.tsx

@@ -0,0 +1,28 @@
+import { forwardRef, InputHTMLAttributes } from 'react'
+import { cn } from '@/lib/utils'
+
+export interface InputProps extends InputHTMLAttributes<HTMLInputElement> {
+  error?: boolean
+}
+
+const Input = forwardRef<HTMLInputElement, InputProps>(
+  ({ className, error, ...props }, ref) => {
+    return (
+      <input
+        className={cn(
+          'flex h-10 w-full rounded-md border bg-white px-3 py-2 text-sm',
+          'placeholder:text-gray-400',
+          'focus:outline-none focus:ring-2 focus:ring-blue-500 focus:ring-offset-1',
+          'disabled:cursor-not-allowed disabled:opacity-50',
+          error ? 'border-red-500' : 'border-gray-300',
+          className
+        )}
+        ref={ref}
+        {...props}
+      />
+    )
+  }
+)
+Input.displayName = 'Input'
+
+export { Input }