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)