feat(ui): queue planning subtasks sequentially and surface waiting status

Adds a "Queue subtasks sequentially" context-menu entry on rows with planning children, wires it to WorkerHub.QueuePlanningSubtasksAsync via IWorkerClient. TaskRowViewModel exposes IsWaiting/StatusChipClass for the new Waiting status, and HasPlanningChildren keeps parents expandable after they leave the planning state. TasksIslandViewModel auto-collapses parents whose every child is Done and includes Waiting children in the queued virtual list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/WorkerClient.cs

@@ -436,6 +436,11 @@ public partial class WorkerClient : ObservableObject, IAsyncDisposable, IWorkerC
         await _hub.InvokeAsync("AbortPlanningMerge", planningTaskId);
     }
 
+    public async Task QueuePlanningSubtasksAsync(string parentTaskId, CancellationToken ct = default)
+    {
+        await _hub.InvokeAsync("QueuePlanningSubtasksAsync", parentTaskId, ct);
+    }
+
     // IWorkerClient explicit implementations (drop typed return values)
     async Task IWorkerClient.StartPlanningSessionAsync(string taskId, CancellationToken ct)
         => await StartPlanningSessionAsync(taskId, ct);