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/State/TaskStateServiceTests.cs

@@ -1,6 +1,7 @@
 using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Planning;
 using ClaudeDo.Worker.State;
 using ClaudeDo.Worker.Tests.Infrastructure;
 using Microsoft.EntityFrameworkCore;
@@ -8,6 +9,12 @@ using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Worker.Tests.State;
 
+file sealed class FakeActiveMergeState : IActiveMergeState
+{
+    public HashSet<string> ActiveTaskIds { get; } = new();
+    public bool HasActiveMerge(string taskId) => ActiveTaskIds.Contains(taskId);
+}
+
 public sealed class TaskStateServiceTests : IDisposable
 {
     private readonly DbFixture _db = new();
@@ -319,6 +326,34 @@ public sealed class TaskStateServiceTests : IDisposable
         Assert.Equal(TaskStatus.Done, await GetStatusAsync(id));
     }
 
+    [Fact]
+    public async Task CancelAsync_WhileUnitMergeDraining_Rejects_AndDoesNotMutate()
+    {
+        var mergeState = new FakeActiveMergeState();
+        var built = TaskStateServiceBuilder.Build(_factory, () => mergeState);
+        var id = await SeedTaskAsync(TaskStatus.WaitingForReview);
+        mergeState.ActiveTaskIds.Add(id);
+
+        var result = await built.State.CancelAsync(id, DateTime.UtcNow, default);
+
+        Assert.False(result.Ok);
+        Assert.Equal(TaskStatus.WaitingForReview, await GetStatusAsync(id));
+    }
+
+    [Fact]
+    public async Task CancelAsync_AfterUnitMergeDrainCompletes_Succeeds()
+    {
+        var mergeState = new FakeActiveMergeState();
+        var built = TaskStateServiceBuilder.Build(_factory, () => mergeState);
+        var id = await SeedTaskAsync(TaskStatus.WaitingForReview);
+        // Merge drain finished (or never started) for this task — cancel behaves as before.
+
+        var result = await built.State.CancelAsync(id, DateTime.UtcNow, default);
+
+        Assert.True(result.Ok);
+        Assert.Equal(TaskStatus.Cancelled, await GetStatusAsync(id));
+    }
+
     // ─── ResetToIdleAsync ─────────────────────────────────────────────────
 
     [Fact]