fix(worker,ui): block cancelling a task while its unit merge is draining

TaskStateService.CancelAsync allowed cancelling a WaitingForReview task
even while PlanningMergeOrchestrator was mid-drain on it: ApproveReview
awaits the whole multi-subtask merge synchronously, so a concurrent
CancelReview (UI or MCP) could flip the parent to Cancelled while the
orchestrator kept merging children onto the target branch, then
FinalizeParentDoneAsync would find the parent no longer WaitingForReview
and give up - leaving the merged diffs stranded with no rollback.

CancelAsync now rejects with a clear reason when HasActiveMerge(taskId)
is true. TaskStateService can't take PlanningMergeOrchestrator as a
direct constructor dependency (circular back to ITaskStateService), so
it takes a lazily-resolved Func<IActiveMergeState> instead, mirroring
the existing Func<ITaskStateService> cycle-break already used for
PlanningChainCoordinator.

UI polish: DetailsIslandViewModel.IsMergeDraining gates
CancelReviewCommand's CanExecute (same shape as
WorktreesOverviewModalViewModel.IsMerging), and the command's catch now
raises ErrorReported instead of swallowing the rejection silently.

src/ClaudeDo.Worker/Program.cs

@@ -101,12 +101,16 @@ builder.Services.AddSingleton<IQueuePicker, QueuePicker>();
 builder.Services.AddSingleton<RunCancellationRegistry>();
 
 builder.Services.AddSingleton<Func<ITaskStateService>>(sp => () => sp.GetRequiredService<ITaskStateService>());
+// PlanningMergeOrchestrator itself depends on ITaskStateService, so TaskStateService can only
+// reach it lazily (Func<IActiveMergeState>) — same cycle-breaking shape as the Func above.
+builder.Services.AddSingleton<Func<IActiveMergeState>>(sp => () => sp.GetRequiredService<PlanningMergeOrchestrator>());
 builder.Services.AddSingleton<ITaskStateService>(sp => new TaskStateService(
     sp.GetRequiredService<IDbContextFactory<ClaudeDoDbContext>>(),
     sp.GetRequiredService<HubBroadcaster>(),
     sp.GetRequiredService<IQueueWaker>(),
     sp.GetRequiredService<PlanningChainCoordinator>(),
     sp.GetRequiredService<RunCancellationRegistry>(),
+    sp.GetRequiredService<Func<IActiveMergeState>>(),
     sp.GetRequiredService<ILogger<TaskStateService>>()));
 
 // Agent file management.