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.BlazorWebApp/Program.cs

@@ -21,11 +21,16 @@ if (!string.IsNullOrWhiteSpace(apiBaseUrl))
     {
         client.BaseAddress = new Uri(apiBaseUrl);
     });
+    builder.Services.AddHttpClient<MassDataApiClient>(client =>
+    {
+        client.BaseAddress = new Uri(apiBaseUrl);
+    });
 }
 else
 {
     builder.Services.AddHttpClient<CatalogApiClient>();
     builder.Services.AddHttpClient<DashboardApiClient>();
+    builder.Services.AddHttpClient<MassDataApiClient>();
 }
 
 var app = builder.Build();