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.Application/MassData/Queries/GetMassDataCountHandler.cs

@@ -0,0 +1,19 @@
+using DbFirst.Application.Repositories;
+using MediatR;
+
+namespace DbFirst.Application.MassData.Queries;
+
+public class GetMassDataCountHandler : IRequestHandler<GetMassDataCountQuery, int>
+{
+    private readonly IMassDataRepository _repository;
+
+    public GetMassDataCountHandler(IMassDataRepository repository)
+    {
+        _repository = repository;
+    }
+
+    public async Task<int> Handle(GetMassDataCountQuery request, CancellationToken cancellationToken)
+    {
+        return await _repository.GetCountAsync(cancellationToken);
+    }
+}

DbFirst.Application/MassData/Queries/GetMassDataCountQuery.cs

@@ -0,0 +1,5 @@
+using MediatR;
+
+namespace DbFirst.Application.MassData.Queries;
+
+public record GetMassDataCountQuery : IRequest<int>;

DbFirst.Application/Repositories/IMassDataRepository.cs

@@ -4,6 +4,7 @@ namespace DbFirst.Application.Repositories;
 
 public interface IMassDataRepository
 {
+    Task<int> GetCountAsync(CancellationToken cancellationToken = default);
     Task<List<Massdata>> GetAllAsync(int? skip = null, int? take = null, CancellationToken cancellationToken = default);
     Task<Massdata?> GetByCustomerNameAsync(string customerName, CancellationToken cancellationToken = default);
     Task<Massdata> UpsertByCustomerNameAsync(string customerName, decimal amount, bool statusFlag, string category, CancellationToken cancellationToken = default);