fix(ui): surface worker-offline failures when deleting a task

DeleteTaskAsync had no IsConnected guard, unlike every other worker-dependent
command in DetailsIslandViewModel, so an offline delete was a silent no-op.
WorkerClient.DeleteTaskAsync also only caught HubException, letting the
InvalidOperationException thrown by an inactive hub connection escape into
the unobserved command task and vanish.

Gate DeleteTaskCommand behind CanDeleteTask (Task != null && IsConnected),
re-evaluate it on connection-state changes, widen WorkerClient to catch the
connection-inactive case too, and wrap the ViewModel's call in try/catch as
a second line of defense against a race between the guard and the call.

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -338,6 +338,11 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         {
             return (false, ex.Message);
         }
+        catch (InvalidOperationException)
+        {
+            // Hub connection is not active (worker offline / not yet connected).
+            return (false, "Worker is offline. Reconnect and try again.");
+        }
     }
 
     public async Task WakeQueueAsync()