fix(client-ui): resolve Medium code-review finding (Client.UI-005)

Call Subscriptions?.Teardown() and Alarms?.Teardown() in the Disconnected
branch of OnConnectionStateChanged so server-side session drops also
quiesce the DataChanged and AlarmEvent handlers. Add Reattach() methods
that idempotently re-hook the handlers; call them from the Connected
branch so reconnects after a server-side drop restore live updates.

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

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

@@ -215,6 +215,16 @@ public partial class AlarmsViewModel : ObservableObject
         ActiveAlarmCount = 0;
     }
 
+    /// <summary>
+    ///     Re-hooks event handlers to the service after a server-side reconnect.
+    ///     Safe to call when already attached (duplicate += is a no-op in .NET multicast delegates).
+    /// </summary>
+    public void Reattach()
+    {
+        _service.AlarmEvent -= OnAlarmEvent;
+        _service.AlarmEvent += OnAlarmEvent;
+    }
+
     /// <summary>
     ///     Unhooks event handlers from the service.
     /// </summary>

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

@@ -166,6 +166,8 @@ public partial class MainWindowViewModel : ObservableObject
         {
             case ConnectionState.Connected:
                 StatusMessage = $"Connected to {EndpointUrl}";
+                Subscriptions?.Reattach();
+                Alarms?.Reattach();
                 break;
             case ConnectionState.Reconnecting:
                 StatusMessage = "Reconnecting...";
@@ -177,6 +179,8 @@ public partial class MainWindowViewModel : ObservableObject
                 StatusMessage = "Disconnected";
                 SessionLabel = string.Empty;
                 RedundancyInfo = null;
+                Subscriptions?.Teardown();
+                Alarms?.Teardown();
                 BrowseTree?.Clear();
                 ReadWrite?.Clear();
                 Subscriptions?.Clear();

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

@@ -265,6 +265,16 @@ public partial class SubscriptionsViewModel : ObservableObject
         SubscriptionCount = 0;
     }
 
+    /// <summary>
+    ///     Re-hooks event handlers to the service after a server-side reconnect.
+    ///     Safe to call when already attached (duplicate += is a no-op in .NET multicast delegates).
+    /// </summary>
+    public void Reattach()
+    {
+        _service.DataChanged -= OnDataChanged;
+        _service.DataChanged += OnDataChanged;
+    }
+
     /// <summary>
     ///     Unhooks event handlers from the service.
     /// </summary>