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.API/Controllers/MassDataController.cs

@@ -17,6 +17,13 @@ public class MassDataController : ControllerBase
         _mediator = mediator;
     }
 
+    [HttpGet("count")]
+    public async Task<ActionResult<int>> GetCount(CancellationToken cancellationToken)
+    {
+        var count = await _mediator.Send(new GetMassDataCountQuery(), cancellationToken);
+        return Ok(count);
+    }
+
     [HttpGet]
     public async Task<ActionResult<IEnumerable<MassDataReadDto>>> GetAll([FromQuery] int? skip, [FromQuery] int? take, CancellationToken cancellationToken)
     {