fix(payment): guard against duplicate package payment verify

Gate payment callback with a semaphore and block double-clicks on checkout
to avoid sending multiple verify requests after gateway redirect.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/FrontOffice.Main/Pages/Checkout.razor.cs

@@ -173,6 +173,8 @@ public partial class Checkout
 
     private async Task ProcessPayment()
     {
+        if (_isProcessingPayment) return;
+
         if (!CanProceedToPayment || _selectedPackage == null || _selectedAddress == null)
         {
             Snackbar.Add("لطفاً پکیج و آدرس را انتخاب کنید.", Severity.Warning);