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.

tests/ClaudeDo.Worker.Tests/Infrastructure/TaskStateServiceBuilder.cs

@@ -20,7 +20,8 @@ public static class TaskStateServiceBuilder
         CountingQueueWaker Waker,
         RunCancellationRegistry RunCancels);
 
-    public static Built Build(IDbContextFactory<ClaudeDoDbContext> dbFactory)
+    public static Built Build(
+        IDbContextFactory<ClaudeDoDbContext> dbFactory, Func<IActiveMergeState>? mergeState = null)
     {
         var hub = new CapturingHubContext();
         var broadcaster = new HubBroadcaster(hub);
@@ -35,12 +36,19 @@ public static class TaskStateServiceBuilder
             waker,
             chain,
             runCancels,
+            mergeState ?? (() => NoActiveMergeState.Instance),
             NullLogger<TaskStateService>.Instance);
 
         return new Built(state, chain, hub, () => waker.Count, waker, runCancels);
     }
 }
 
+file sealed class NoActiveMergeState : IActiveMergeState
+{
+    public static readonly NoActiveMergeState Instance = new();
+    public bool HasActiveMerge(string taskId) => false;
+}
+
 public sealed class CountingQueueWaker : IQueueWaker
 {
     private int _count;