fix(adminui): fleet page disposes its poll timer asynchronously like hosts

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Fleet.razor

@@ -12,7 +12,7 @@
 @using ZB.MOM.WW.OtOpcUa.Configuration.Enums
 @inject IClusterRoleInfo Cluster
 @inject IDbContextFactory<OtOpcUaConfigDbContext> DbFactory
-@implements IDisposable
+@implements IAsyncDisposable
 
 <div class="d-flex justify-content-between align-items-center mb-3">
     <h4 class="mb-0">Fleet status</h4>
@@ -110,8 +110,8 @@ else
     // next one. A manual Refresh landing on top of an in-flight tick is a no-op by the same guard.
     private int _tickRunning;
 
-    // Page-lifetime cancellation: cancelled in Dispose() so a tick in flight when the circuit goes
+    // Page-lifetime cancellation: cancelled in DisposeAsync() so a tick in flight when the circuit
-    // away aborts its query instead of running it to completion against a component that's gone.
+    // goes away aborts its query instead of running it to completion against a component that's gone.
     private readonly CancellationTokenSource _cts = new();
 
     protected override async Task OnInitializedAsync()
@@ -184,12 +184,14 @@ else
 
     private static string StatusLabel(NodeDeploymentStatus? status) => status?.ToString() ?? "waiting";
 
-    public void Dispose()
+    public async ValueTask DisposeAsync()
     {
-        // Cancel BEFORE disposing the timer so a tick already in flight aborts its query rather than
+        // Cancel before draining so a tick already in flight aborts its queries rather than running
-        // running to completion; the CTS is disposed last, once nothing new can observe the token.
+        // them to completion against a component that's gone.
         _cts.Cancel();
-        _timer?.Dispose();
+        // Drain the timer so an in-flight callback can't touch a component that's already gone
+        // (System.Threading.Timer's async dispose awaits any in-flight callback — .NET 6+).
+        if (_timer is not null) await _timer.DisposeAsync();
         _cts.Dispose();
     }