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/Store/ProductDetail.razor.cs

@@ -12,6 +12,7 @@ public partial class ProductDetail : ComponentBase, IDisposable
     [Inject] private ProductService ProductService { get; set; } = default!;
     [Inject] private CartService Cart { get; set; } = default!;
     [Inject] private VATService VAT { get; set; } = default!;
+    [Inject] private GuestActionGate GuestGate { get; set; } = default!;
 
     [Parameter] public long id { get; set; }
 
@@ -92,14 +93,18 @@ public partial class ProductDetail : ComponentBase, IDisposable
     private async Task AddToCart()
     {
         if (_product is null) return;
-        await Cart.Add(_product, 1);
+        var product = _product;
+        await GuestGate.RunAsync(() => Cart.Add(product, 1));
     }
 
     private async Task RemoveFromCart()
     {
         if (_product is null) return;
-        _qty--;
-        await Cart.UpdateQuantity(CurrentCartItem.ProductId, _qty);
+        await GuestGate.RunAsync(async () =>
+        {
+            _qty--;
+            await Cart.UpdateQuantity(CurrentCartItem!.ProductId, _qty);
+        });
     }
 
     private void IncreaseLocalQty()