fix(client-ui): resolve Medium code-review finding (Client.UI-008)

Implement IDisposable on MainWindowViewModel to detach ConnectionStateChanged,
call Teardown() on the subscription/alarm VMs, and dispose _service so the OPC UA
session and SDK resources are released. Call Dispose() from MainWindow.OnClosing
alongside the existing SaveSettings() call.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/ViewModels/MainWindowViewModel.cs

@@ -10,7 +10,7 @@ namespace ZB.MOM.WW.OtOpcUa.Client.UI.ViewModels;
 /// <summary>
 ///     Main window ViewModel coordinating all panels.
 /// </summary>
-public partial class MainWindowViewModel : ObservableObject
+public partial class MainWindowViewModel : ObservableObject, IDisposable
 {
     private readonly IUiDispatcher _dispatcher;
     private readonly IOpcUaClientServiceFactory _factory;
@@ -411,6 +411,21 @@ public partial class MainWindowViewModel : ObservableObject
         });
     }
 
+    /// <summary>
+    /// Detaches the connection-state handler and disposes the OPC UA client service, releasing
+    /// the session, certificate validator, and any background reconnect resources.
+    /// </summary>
+    public void Dispose()
+    {
+        if (_service != null)
+        {
+            _service.ConnectionStateChanged -= OnConnectionStateChanged;
+            Subscriptions?.Teardown();
+            Alarms?.Teardown();
+            _service.Dispose();
+        }
+    }
+
     private static string[]? ParseFailoverUrls(string? csv)
     {
         if (string.IsNullOrWhiteSpace(csv))

src/Client/ZB.MOM.WW.OtOpcUa.Client.UI/Views/MainWindow.axaml.cs

@@ -140,7 +140,10 @@ public partial class MainWindow : Window
     protected override void OnClosing(WindowClosingEventArgs e)
     {
         if (DataContext is MainWindowViewModel vm)
+        {
             vm.SaveSettings();
+            vm.Dispose();
+        }
         base.OnClosing(e);
     }
 }