Phase 8a: Wire up Checkout page to Customer* RPCs

- Checkout.razor.cs: Rewrite ProcessPayment() to use
  CustomerPurchasePackageAsync instead of disabled dead code
  (old TransactionsContract + UserOrderContract flow removed)
- Callback URL now points to /profile/payment-callback
  (reuses existing verify page with CustomerVerifyPackagePurchase)
- Remove unused DI: UserOrderContract, TransactionContract
- Remove unused usings: Transactions, UserOrder, WellKnownTypes
- Profile/Index.razor.cs: Rename basePackage → selectedPackage,
  tempCallbackUrl → callbackUrl (cosmetic)
- Build: 0 errors

src/FrontOffice.Main/Pages/Profile/Index.razor.cs

@@ -413,23 +413,22 @@ public partial class Index
         
         try
         {
-            // Get available packages and pick the first (base) one
+            // Get available packages and pick the first one
             var packages = await PackageContract.GetCustomerPackagesAsync(new GetCustomerPackagesRequest());
-            var basePackage = packages?.Models?.FirstOrDefault();
-            if (basePackage == null)
+            var selectedPackage = packages?.Models?.FirstOrDefault();
+            if (selectedPackage == null)
             {
                 Snackbar.Add("پکیجی برای خرید یافت نشد", Severity.Error);
                 return;
             }
             
             // Call CMS to initiate purchase (UserId from JWT, PackageId explicit)
-            // First, get orderId — then embed in callback URL
-            var tempCallbackUrl = $"{Navigation.BaseUri}profile/payment-callback";
+            var callbackUrl = $"{Navigation.BaseUri}profile/payment-callback";
             
             var response = await PackageContract.CustomerPurchasePackageAsync(new CustomerPurchasePackageRequest
             {
-                PackageId = basePackage.Id,
-                CallbackUrl = tempCallbackUrl,
+                PackageId = selectedPackage.Id,
+                CallbackUrl = callbackUrl,
                 PurchaseMethod = PurchaseMethodEnum.PurchaseMethodGateway
             });