Refactor API clients to use interface-based DI

Introduce interfaces for all API clients and update dependency injection to use these interfaces. Refactor services and components to depend on abstractions instead of concrete implementations, improving testability and maintainability.

DbFirst.BlazorWebApp/Services/IMassDataApiClient.cs

@@ -0,0 +1,12 @@
+using DbFirst.BlazorWebApp.Models;
+
+namespace DbFirst.BlazorWebApp.Services
+{
+    public interface IMassDataApiClient
+    {
+        Task<int> GetCountAsync();
+        Task<List<MassDataReadDto>> GetAllAsync(int? skip, int? take);
+        Task<ApiResult<MassDataReadDto?>> UpsertAsync(MassDataWriteDto dto);
+        Task<MassDataReadDto?> GetByCustomerNameAsync(string customerName);
+    }
+}