fix(ui): default-expand diff tree; reliable row-click toggle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/Modals/WorktreeModalViewModel.cs

@@ -20,6 +20,7 @@ public sealed partial class WorktreeNodeViewModel : ViewModelBase
     public bool IsDirectory { get; init; }
     public string RelativePath { get; init; } = "";
     public ObservableCollection<WorktreeNodeViewModel> Children { get; } = new();
+    [ObservableProperty] private bool _isExpanded = true;
 }
 
 public sealed partial class WorktreeModalViewModel : ViewModelBase