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/Checkout.razor.cs

@@ -16,7 +16,6 @@ public partial class Checkout
     private bool _loadingAddresses;
 
     private long _discountBalance;
-    private long _discountBalanceToUse;
     private string? _notes;
     private bool _placing;
 
@@ -27,13 +26,14 @@ public partial class Checkout
 
     /// <summary>
     /// Maximum the user can actually use = min(balance, allowed discount)
+    /// Always applied at 100% — no user selection
     /// </summary>
     private long MaxUsable => Math.Min(_discountBalance, MaxAllowedDiscount);
 
     /// <summary>
-    /// Amount to be charged via payment gateway
+    /// Amount to be charged via payment gateway (total minus auto-applied discount)
     /// </summary>
-    private long GatewayAmount => DiscountCart.TotalPrice - _discountBalanceToUse;
+    private long GatewayAmount => DiscountCart.TotalPrice - MaxUsable;
 
     private bool CanPlaceOrder => DiscountCart.Items.Count > 0 && _selectedAddress is not null;
 
@@ -44,9 +44,6 @@ public partial class Checkout
         var addressTask = LoadAddresses();
         var balanceTask = LoadDiscountBalance();
         await Task.WhenAll(addressTask, balanceTask);
-
-        // Default: use maximum possible discount
-        _discountBalanceToUse = MaxUsable;
     }
 
     private async Task LoadDiscountBalance()
@@ -101,12 +98,10 @@ public partial class Checkout
         _placing = true;
         try
         {
-            // Clamp the discount balance to use
-            var discountToUse = Math.Min(_discountBalanceToUse, MaxUsable);
-
+            // Always use maximum possible discount (100%)
             var result = await DiscountOrderService.PlaceOrderAsync(
                 _selectedAddress.Id,
-                discountToUse,
+                MaxUsable,
                 _notes);
 
             if (!result.Success)