feat: add VAT display to all DiscountStore pages — Products (note), ProductDetail (breakdown), Cart (summary line), Checkout (refactor to VATService), OrderDetail (financial summary)

src/FrontOffice.Main/Pages/DiscountStore/OrderDetail.razor.cs

@@ -12,12 +12,28 @@ public partial class OrderDetail
     public string? PaymentResult { get; set; }
 
     [Inject] private DiscountOrderService DiscountOrderService { get; set; } = default!;
+    [Inject] private VATService VAT { get; set; } = default!;
 
     private DiscountOrderDetail? _order;
     private bool _loading = true;
     private string? _paymentMessage;
     private MudBlazor.Severity _paymentSeverity;
 
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (firstRender)
+        {
+            await VAT.LoadAsync();
+            StateHasChanged();
+        }
+    }
+
+    /// <summary>مبلغ قبل از مالیات (جمع کل - اعتبار)</summary>
+    private long PreVatGateway => _order is null ? 0 : (_order.TotalPrice - _order.DiscountBalanceUsed);
+
+    /// <summary>مالیات بر ارزش افزوده</summary>
+    private long OrderVatAmount => VAT.CalculateVAT(PreVatGateway);
+
     protected override async Task OnInitializedAsync()
     {
         // نمایش پیام نتیجه پرداخت (بعد از بازگشت از درگاه)