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/auth/google-button.tsx

@@ -0,0 +1,68 @@
+'use client'
+
+import { createClient } from '@/lib/supabase/client'
+import { Button } from '@/components/ui/button'
+import { useState } from 'react'
+
+// Simple Google icon SVG
+function GoogleIcon({ className }: { className?: string }) {
+  return (
+    <svg className={className} viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg">
+      <path
+        d="M22.56 12.25c0-.78-.07-1.53-.2-2.25H12v4.26h5.92c-.26 1.37-1.04 2.53-2.21 3.31v2.77h3.57c2.08-1.92 3.28-4.74 3.28-8.09z"
+        fill="#4285F4"
+      />
+      <path
+        d="M12 23c2.97 0 5.46-.98 7.28-2.66l-3.57-2.77c-.98.66-2.23 1.06-3.71 1.06-2.86 0-5.29-1.93-6.16-4.53H2.18v2.84C3.99 20.53 7.7 23 12 23z"
+        fill="#34A853"
+      />
+      <path
+        d="M5.84 14.09c-.22-.66-.35-1.36-.35-2.09s.13-1.43.35-2.09V7.07H2.18C1.43 8.55 1 10.22 1 12s.43 3.45 1.18 4.93l2.85-2.22.81-.62z"
+        fill="#FBBC05"
+      />
+      <path
+        d="M12 5.38c1.62 0 3.06.56 4.21 1.64l3.15-3.15C17.45 2.09 14.97 1 12 1 7.7 1 3.99 3.47 2.18 7.07l3.66 2.84c.87-2.6 3.3-4.53 6.16-4.53z"
+        fill="#EA4335"
+      />
+    </svg>
+  )
+}
+
+export function GoogleSignInButton() {
+  const [loading, setLoading] = useState(false)
+  const supabase = createClient()
+
+  async function handleGoogleSignIn() {
+    setLoading(true)
+
+    const { error } = await supabase.auth.signInWithOAuth({
+      provider: 'google',
+      options: {
+        redirectTo: `${window.location.origin}/auth/callback`,
+        queryParams: {
+          access_type: 'offline',
+          prompt: 'consent',
+        },
+      },
+    })
+
+    if (error) {
+      console.error('Google sign-in error:', error)
+      setLoading(false)
+    }
+    // Note: No need to handle success - user is redirected to Google
+  }
+
+  return (
+    <Button
+      type="button"
+      variant="outline"
+      onClick={handleGoogleSignIn}
+      disabled={loading}
+      className="w-full flex items-center justify-center gap-2"
+    >
+      <GoogleIcon className="w-5 h-5" />
+      {loading ? 'Reindirizzamento...' : 'Accedi con Google'}
+    </Button>
+  )
+}

src/components/ui/button.tsx

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+import { forwardRef, ButtonHTMLAttributes } from 'react'
+import { cn } from '@/lib/utils'
+
+export interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: 'default' | 'outline' | 'ghost'
+  size?: 'default' | 'sm' | 'lg'
+}
+
+const Button = forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ className, variant = 'default', size = 'default', ...props }, ref) => {
+    return (
+      <button
+        className={cn(
+          'inline-flex items-center justify-center rounded-md font-medium transition-colors',
+          'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-offset-2',
+          'disabled:pointer-events-none disabled:opacity-50',
+          {
+            'bg-blue-600 text-white hover:bg-blue-700': variant === 'default',
+            'border border-gray-300 bg-white hover:bg-gray-50': variant === 'outline',
+            'hover:bg-gray-100': variant === 'ghost',
+          },
+          {
+            'h-10 px-4 py-2': size === 'default',
+            'h-8 px-3 text-sm': size === 'sm',
+            'h-12 px-6 text-lg': size === 'lg',
+          },
+          className
+        )}
+        ref={ref}
+        {...props}
+      />
+    )
+  }
+)
+Button.displayName = 'Button'
+
+export { Button }

src/components/ui/card.tsx

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+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 }

src/lib/utils.ts

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