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.Infrastructure/Repositories/MassDataRepository.cs

@@ -15,6 +15,11 @@ public class MassDataRepository : IMassDataRepository
         _db = db;
     }
 
+    public async Task<int> GetCountAsync(CancellationToken cancellationToken = default)
+    {
+        return await _db.Massdata.AsNoTracking().CountAsync(cancellationToken);
+    }
+
     public async Task<List<Massdata>> GetAllAsync(CancellationToken cancellationToken = default)
     {
         return await _db.Massdata.AsNoTracking().ToListAsync(cancellationToken);