Add initial empty DTO classes for Catalogs, Dashboards, etc.

Introduced six new internal DTO classes: CatalogReadDto, CatalogWriteDto, DashboardInfoDto, LayoutDto, MassdataReadDto, and MassdataWriteDto. Each class resides in its appropriate namespace and currently contains no properties or methods, serving as placeholders for future data transfer logic.

DbFirst.Contracts/Massdata/MassdataWriteDto.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DbFirst.Contracts.Massdata
+{
+    internal class MassdataWriteDto
+    {
+    }
+}