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/Utilities/ProductService.cs

@@ -71,12 +71,12 @@ public class ProductService
         return result.Products;
     }
 
-    public Task<ProductListResult> GetTopSellingAsync(int count = 6)
-        => GetProductsPagedAsync(sortBy: "SaleCount desc", page: 1, pageSize: count);
+    public Task<ProductListResult> GetTopSellingAsync(int count = 6, bool? inStock = null)
+        => GetProductsPagedAsync(sortBy: "SaleCount desc", page: 1, pageSize: count, inStock: inStock);
 
     public async Task<ProductListResult> GetProductsPagedAsync(
         string? query = null, long? categoryId = null, string? sortBy = null,
-        int page = 1, int pageSize = 12)
+        int page = 1, int pageSize = 12, bool? inStock = null)
     {
         try
         {
@@ -87,20 +87,25 @@ public class ProductService
                     PageNumber = page,
                     PageSize = pageSize
                 },
-                Filter = new GetAllProductsByFilterFilter
-                {
-                    Title = query ?? string.Empty,
-                    Description = query ?? string.Empty,
-                    ShortInfomation = query ?? string.Empty,
-                    FullInformation = query ?? string.Empty
-                }
+                Filter = new GetAllProductsByFilterFilter()
             };
 
+            // فقط Title — CMS فیلترها را AND می‌زند؛ ارسال query در همه فیلدها جستجو را می‌شکند
+            if (!string.IsNullOrWhiteSpace(query))
+            {
+                request.Filter.Title = query.Trim();
+            }
+
             if (categoryId is { } value)
             {
                 request.Filter.CategoryId = value;
             }
 
+            if (inStock.HasValue)
+            {
+                request.Filter.InStock = inStock.Value;
+            }
+
             if (!string.IsNullOrEmpty(sortBy))
             {
                 request.SortBy = sortBy;