Refactor CartService to use async methods for cart operations and integrate gRPC for backend synchronization

src/FrontOffice.Main/Pages/Store/ProductDetail.razor.cs

@@ -21,10 +21,10 @@ public partial class ProductDetail : ComponentBase
         _loading = false;
     }
 
-    private void AddToCart()
+    private async Task AddToCart()
     {
         if (_product is null) return;
-        Cart.Add(_product, _qty);
+        await Cart.Add(_product, _qty);
     }
 
     private static string FormatPrice(long price) => string.Format("{0:N0} تومان", price);