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/Services/MassDataApiClient.cs

@@ -0,0 +1,35 @@
+using System.Net.Http.Json;
+using DbFirst.BlazorWebApp.Models;
+
+namespace DbFirst.BlazorWebApp.Services;
+
+public class MassDataApiClient
+{
+    private readonly HttpClient _httpClient;
+    private const string Endpoint = "api/massdata";
+
+    public MassDataApiClient(HttpClient httpClient)
+    {
+        _httpClient = httpClient;
+    }
+
+    public async Task<int> GetCountAsync()
+    {
+        var result = await _httpClient.GetFromJsonAsync<int?>("api/massdata/count");
+        return result ?? 0;
+    }
+
+    public async Task<List<MassDataReadDto>> GetAllAsync(int skip, int take)
+    {
+        var result = await _httpClient.GetFromJsonAsync<List<MassDataReadDto>>($"{Endpoint}?skip={skip}&take={take}");
+        return result ?? new List<MassDataReadDto>();
+    }
+
+    public async Task<MassDataReadDto> UpsertAsync(MassDataWriteDto dto)
+    {
+        var response = await _httpClient.PostAsJsonAsync($"{Endpoint}/upsert", dto);
+        response.EnsureSuccessStatusCode();
+        var payload = await response.Content.ReadFromJsonAsync<MassDataReadDto>();
+        return payload ?? new MassDataReadDto();
+    }
+}