feat: force 100% discount — remove slider, auto-apply max discount from wallet

- Remove MudSlider and MudNumericField from Checkout
- Auto-apply MaxUsable (full discount balance)
- Show fixed '100% تخفیفی' badge on Products, Cart, ProductDetail, Checkout
- GatewayAmount = TotalPrice - MaxUsable (no user choice)

src/FrontOffice.Main/Pages/DiscountStore/Cart.razor

@@ -40,7 +40,7 @@
                             <MudTd>@FormatPrice(context.UnitPrice)</MudTd>
                             <MudTd>
                                 <MudChip T="string" Color="Color.Error" Variant="Variant.Outlined" Size="Size.Small">
-                                    تا @context.MaxDiscountPercent%
+                                    ۱۰۰٪ تخفیفی
                                 </MudChip>
                             </MudTd>
                             <MudTd>
@@ -86,7 +86,7 @@
                                         <MudText Typo="Typo.subtitle2">@item.Title</MudText>
                                     </MudStack>
                                     <MudChip T="string" Color="Color.Error" Variant="Variant.Outlined" Size="Size.Small">
-                                        تا @item.MaxDiscountPercent%
+                                        ۱۰۰٪ تخفیفی
                                     </MudChip>
                                 </MudStack>