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.BlazorWasm/Program.cs

@@ -18,5 +18,6 @@ var apiBaseUrl = builder.Configuration["ApiBaseUrl"] ?? builder.HostEnvironment.
 builder.Services.AddScoped(sp => new HttpClient { BaseAddress = new Uri(apiBaseUrl) });
 builder.Services.AddScoped<CatalogApiClient>();
 builder.Services.AddScoped<DashboardApiClient>();
+builder.Services.AddScoped<MassDataApiClient>();
 
 await builder.Build().RunAsync();