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/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);