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/Checkout.razor.cs

@@ -11,6 +11,8 @@ public partial class Checkout
     [Inject] private DiscountOrderService DiscountOrderService { get; set; } = default!;
     [Inject] private UserAddressContract.UserAddressContractClient UserAddressContract { get; set; } = default!;
     [Inject] private VATService VAT { get; set; } = default!;
+    [Inject] private AuthDialogService AuthDialogService { get; set; } = default!;
+    [Inject] private AuthService AuthService { get; set; } = default!;
 
     private List<CustomerAddressModel> _addresses = new();
     private CustomerAddressModel? _selectedAddress;
@@ -32,6 +34,10 @@ public partial class Checkout
 
     protected override async Task OnInitializedAsync()
     {
+        if (!await AuthService.IsAuthenticatedAsync())
+        {
+            await AuthDialogService.ShowAuthDialogAsync();
+        }
         await VAT.LoadAsync();
         await DiscountCart.EnsureInitializedAsync();
         await LoadAddresses();