fix: Add missing currency field to checkout items tracking

CartItemData requires currency field for each item.
Added currency extraction from variant pricing.

src/app/[locale]/checkout/page.tsx

@@ -183,6 +183,7 @@ export default function CheckoutPage() {
           name: line.variant.product.name,
           quantity: line.quantity,
           price: line.variant.pricing?.price?.gross?.amount || 0,
+          currency: line.variant.pricing?.price?.gross?.currency || "RSD",
         })),
       });
     }