fix(cart): use session-based auth instead of localStorage for checkout

Cart page checked localStorage.getItem('user') which is never set by
the cookie-based auth system. Replaced with useUser() context and
corrected redirect target from /cart to /checkout.

app/src/app/cart/page.tsx

@@ -5,6 +5,7 @@ import { useRouter } from 'next/navigation'
 import Link from 'next/link'
 import { Navbar } from '@/components/storefront/Navbar'
 import { Button } from '@/components/ui/Button'
+import { useUser } from '@/context/UserContext'
 
 interface CartItem {
   productId: string
@@ -18,6 +19,7 @@ interface CartItem {
 export default function CartPage() {
   const [cart, setCart] = useState<CartItem[]>([])
   const router = useRouter()
+  const { user } = useUser()
 
   useEffect(() => {
     const stored = JSON.parse(localStorage.getItem('cart') || '[]')
@@ -51,10 +53,9 @@ export default function CartPage() {
 
   const subtotal = cart.reduce((sum, item) => sum + item.price * item.quantity, 0)
 
-  async function handleCheckout() {
-    const user = localStorage.getItem('user')
+  function handleCheckout() {
     if (!user) {
-      router.push('/login?redirect=/cart')
+      router.push('/login?redirect=/checkout')
       return
     }
     router.push('/checkout')