feat: update product service to support in-stock filtering and enhance top-selling product retrieval

- Modified `GetTopSellingAsync` methods in `ProductService` and `DiscountProductService` to include an optional `inStock` parameter for filtering products based on availability.
- Introduced a constant for the top product count in `Index.razor.cs` to improve maintainability and readability.
- Updated the product retrieval logic in `Index.razor.cs` to utilize the new in-stock filtering feature.

These changes enhance the product listing functionality by allowing users to view only in-stock top-selling products, improving the overall shopping experience.

src/FrontOffice.Main/Pages/Index.razor.cs

@@ -24,6 +24,8 @@ public partial class Index : IDisposable
     private LandingSettings? _settings;
 
     // ── Top-selling product sections ──
+    private const int LandingTopProductCount = 6;
+
     private List<Product> _topRegularProducts = new();
     private List<DiscountProductCard> _topDiscountProducts = new();
     private bool _loadingTopProducts = true;
@@ -65,9 +67,9 @@ public partial class Index : IDisposable
         PopulateFromSettings();
         _dataLoaded = true;
 
-        // Load top-selling products and blog posts in parallel
-        var topRegTask = ProductService.GetTopSellingAsync(6);
-        var topDiscTask = DiscountProductService.GetTopSellingAsync(6);
+        // Load top-selling in-stock products and blog posts in parallel
+        var topRegTask = ProductService.GetTopSellingAsync(LandingTopProductCount, inStock: true);
+        var topDiscTask = DiscountProductService.GetTopSellingAsync(LandingTopProductCount, inStock: true);
         var featuredPostsTask = BlogPostService.GetFeaturedPostsAsync(2);
 
         try