Add MassData feature with API, paging, and Blazor grid

Introduces MassData management to backend and Blazor frontend:
- Adds API endpoint for MassData count and paging
- Updates repository and controller for count support
- Implements MediatR query/handler for count
- Adds Blazor page and grid for viewing/editing MassData
- Registers MassDataApiClient and integrates with DI
- Supports paging, upsert, and UI feedback in grid

DbFirst.BlazorWebApp/Components/Pages/Massdata.razor

@@ -0,0 +1,7 @@
+@page "/massdata"
+
+<PageTitle>MassData</PageTitle>
+
+<h1>MassData</h1>
+
+<MassDataGrid />