fix(dashboard): parallel bounded alarm drains, best-effort disposal catch-alls, ConfigureAwait alignment

docs/GatewayDashboardDesign.md

@@ -246,9 +246,11 @@ the pill therefore reports is the case it exists for — the channel completing
 a page that is still watching.
 
 `AlarmsPage` owns two loops of its own (the 3 s alarm poll and the provider-status
-badge) and bounds each drain at 5 seconds on dispose, for the same reason
+badge) and bounds their drain at 5 seconds on dispose, for the same reason
 `DashboardPageBase` bounds its watch drain: both loops render through the renderer's
-dispatcher, and disposal can run on it.
+dispatcher, and disposal can run on it. The two are drained concurrently, so the
+bound on disposal is 5 seconds in total rather than per loop — a wedged dispatcher
+blocks both loops at once, and draining them in sequence would time out twice.
 
 ### SignalR hubs (remote clients)
 

src/ZB.MOM.WW.MxGateway.Server/Dashboard/Components/Pages/AlarmsPage.razor

@@ -323,6 +323,14 @@
         catch (OperationCanceledException)
         {
         }
+        catch
+        {
+            // Catch-all for the same reason ProviderStatusLoopAsync has one: a teardown race
+            // can fault InvokeAsync (a disposed renderer) after cancellation has already been
+            // requested. Letting that fault the task would surface it out of the drain in
+            // DisposeAsync, skipping _cts.Dispose(). The loop ends here and the page holds its
+            // last rendered rows — it is being disposed or has nothing left to poll with.
+        }
     }
 
     private async Task RefreshAlarmsAsync()
@@ -341,10 +349,13 @@
     /// <inheritdoc />
     public async ValueTask DisposeAsync()
     {
-        await _cts.CancelAsync();
+        await _cts.CancelAsync().ConfigureAwait(false);
 
-        await DrainAsync(_pollTask);
-        await DrainAsync(_providerStatusTask);
+        // Drained together, not one after the other: the wedged dispatcher this bound exists
+        // for blocks both loops at once, so sequential drains would time out twice and make
+        // the real bound 10 seconds. DrainAsync tolerates a null task.
+        await Task.WhenAll(DrainAsync(_pollTask), DrainAsync(_providerStatusTask))
+            .ConfigureAwait(false);
 
         _cts.Dispose();
         GC.SuppressFinalize(this);
@@ -362,7 +373,7 @@
 
         try
         {
-            await loop.WaitAsync(LoopDrainTimeout);
+            await loop.WaitAsync(LoopDrainTimeout).ConfigureAwait(false);
         }
         catch (TimeoutException)
         {
@@ -370,5 +381,9 @@
         catch (OperationCanceledException)
         {
         }
+        catch
+        {
+            // Other disposal-time errors are best-effort.
+        }
     }
 }