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/Utilities/DiscountProductService.cs

@@ -80,7 +80,7 @@ public class DiscountProductService
     public async Task<DiscountProductListResult> GetProductsAsync(
         int page = 1, int pageSize = 12,
         string? search = null, long? categoryId = null,
-        bool? inStock = null)
+        bool? inStock = null, string? sortBy = null)
     {
         try
         {
@@ -97,6 +97,8 @@ public class DiscountProductService
                 request.CategoryId = categoryId.Value;
             if (inStock.HasValue)
                 request.InStock = inStock.Value;
+            if (!string.IsNullOrWhiteSpace(sortBy))
+                request.SortBy = sortBy;
 
             var response = await _productClient.GetDiscountProductsAsync(request);
 
@@ -125,6 +127,9 @@ public class DiscountProductService
         }
     }
 
+    public Task<DiscountProductListResult> GetTopSellingAsync(int count = 6)
+        => GetProductsAsync(page: 1, pageSize: count, sortBy: "SaleCount desc");
+
     public async Task<DiscountProductDetail?> GetByIdAsync(long productId)
     {
         try