fix(adminui): dispose Alerts CTS + self-render ShowOpResult (T21 review)

Dispose the CancellationTokenSource in AcknowledgeAsync and ShelveAsync
(the TimeSpan overload holds an internal timer — leaked without using).
Add StateHasChanged() to ShowOpResult so the result chip renders even if
a future caller omits the finally-block re-render.

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

@@ -147,9 +147,9 @@ else
         try
         {
             var user = await GetCurrentUserNameAsync();
+            using var cts = new System.Threading.CancellationTokenSource(TimeSpan.FromSeconds(15));
             var result = await AdminOps.AcknowledgeAlarmAsync(
-                alarmId, user, comment: null,
-                new System.Threading.CancellationTokenSource(TimeSpan.FromSeconds(15)).Token);
+                alarmId, user, comment: null, cts.Token);
             ShowOpResult(alarmId, result.Ok, result.Ok ? "Ack sent" : (result.Message ?? "Failed"));
         }
         catch (Exception ex)
@@ -174,9 +174,9 @@ else
             // Timed shelve (with an unshelve-at datetime picker) is deferred — only OneShot + Unshelve
             // are surfaced here, so unshelveAtUtc is always null. TimedShelve is fully wired through the
             // singleton + AlarmCommand if a UI is added later.
+            using var cts = new System.Threading.CancellationTokenSource(TimeSpan.FromSeconds(15));
             var result = await AdminOps.ShelveAlarmAsync(
-                alarmId, user, kind, unshelveAtUtc: null, comment: null,
-                new System.Threading.CancellationTokenSource(TimeSpan.FromSeconds(15)).Token);
+                alarmId, user, kind, unshelveAtUtc: null, comment: null, cts.Token);
             var verb = kind == ShelveKind.Unshelve ? "Unshelve" : "Shelve";
             ShowOpResult(alarmId, result.Ok, result.Ok ? $"{verb} sent" : (result.Message ?? "Failed"));
         }
@@ -209,6 +209,7 @@ else
         _opResultAlarmId = alarmId;
         _opResultOk = ok;
         _opResultMessage = message;
+        StateHasChanged();
     }
 
     private void OnAlarm(AlarmTransitionEvent evt) =>