fix(ui): surface precondition hints for silently-disabled review buttons (UX-Audit #2)

Cancel now shows an amber hint + tooltip while a merge drain is in
progress (ShowMergeDrainHint mirrors IsMergeDraining). Continue,
Reset & Retry, Open Diff, Worktree, and Review Combined Diff gain
ToolTip.Tip + ToolTip.ShowOnDisabled explaining why they're locked.
No CanExecute or behavior changed, only added communication.

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -1364,8 +1364,13 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     // fix); this just keeps the button from inviting a click the worker will reject anyway.
     [ObservableProperty]
     [NotifyCanExecuteChangedFor(nameof(CancelReviewCommand))]
+    [NotifyPropertyChangedFor(nameof(ShowMergeDrainHint))]
     private bool _isMergeDraining;
 
+    // Drives the visible hint next to Cancel while a merge drain is in progress, so the
+    // silently-disabled button doesn't look broken.
+    public bool ShowMergeDrainHint => IsMergeDraining;
+
     [RelayCommand(CanExecute = nameof(CanCancelReview))]
     private async System.Threading.Tasks.Task CancelReviewAsync()
     {