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/Components/Layout/NavMenu.razor

@@ -36,6 +36,12 @@
                 <span class="oi oi-list-rich" aria-hidden="true"></span> Dashboards
             </NavLink>
         </div>
+
+        <div class="nav-item px-3">
+            <NavLink class="nav-link" href="massdata">
+                <span class="bi bi-table" aria-hidden="true"></span> MassData
+            </NavLink>
+        </div>
     </nav>
 </div>