feat: add top-seller product sections to landing page with guest browsing support

- Add two sections to landing page (Index.razor) below the hero:
  - Top 6 best-selling regular products (3-per-row grid)
  - Top 6 best-selling discount store products (3-per-row grid)
  - Each section has a 'More' button linking to /products and /discount-store
- Add GuestActionGate utility service: wraps auth-required actions; shows
  login modal for guests and resumes the original action after successful login
- Register GuestActionGate as scoped service in ConfigureServices
- Add GetTopSellingAsync(count) to ProductService and DiscountProductService
- Add optional sortBy parameter to DiscountProductService.GetProductsAsync
- Hybridize all product pages for guest browsing:
  - Store/Products, Store/ProductDetail: wrap AddToCart with GuestActionGate
  - DiscountStore/Products, DiscountStore/ProductDetail: wrap AddToCart with GuestActionGate
  - Store/Cart, DiscountStore/Cart, Store/CheckoutSummary: soft auth gate on page init
- Fix MembershipPage membership benefit text to be package-agnostic

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

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

@@ -10,6 +10,7 @@ public partial class ProductDetail
     [Inject] private DiscountProductService DiscountProductService { get; set; } = default!;
     [Inject] private DiscountCartService DiscountCartService { get; set; } = default!;
     [Inject] private VATService VAT { get; set; } = default!;
+    [Inject] private GuestActionGate GuestGate { get; set; } = default!;
 
     private DiscountProductDetail? _product;
     private string _selectedImage = string.Empty;
@@ -59,8 +60,14 @@ public partial class ProductDetail
         _addingToCart = true;
         try
         {
-            await DiscountCartService.AddAsync(_product.Id, _quantity);
-            Snackbar.Add($"{_product.Title} به سبد خرید اضافه شد", MudBlazor.Severity.Success);
+            var productId = _product.Id;
+            var qty = _quantity;
+            var title = _product.Title;
+            await GuestGate.RunAsync(async () =>
+            {
+                await DiscountCartService.AddAsync(productId, qty);
+                Snackbar.Add($"{title} به سبد خرید اضافه شد", MudBlazor.Severity.Success);
+            });
         }
         catch
         {