feat(ui): add subtask tree view with expand/collapse in task list

Tasks with subtasks show a chevron for inline expand/collapse.
Subtask checkboxes toggle completion state directly. Also sets
Windows AppUserModelID for proper taskbar identity.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/TaskListView.axaml.cs

@@ -1,3 +1,4 @@
+using System.Collections.ObjectModel;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Interactivity;
@@ -97,6 +98,29 @@ public partial class TaskListView : UserControl
         }
     }
 
+    private void OnSubtaskPointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(null).Properties.IsRightButtonPressed
+            && sender is Control { DataContext: SubtaskItemViewModel subtask }
+            && DataContext is TaskListViewModel vm)
+        {
+            var parent = vm.Tasks.FirstOrDefault(t => t.Subtasks.Contains(subtask));
+            if (parent is not null)
+                vm.SelectedTask = parent;
+        }
+    }
+
+    private async void OnSubtaskCheckboxClick(object? sender, RoutedEventArgs e)
+    {
+        if (sender is CheckBox { DataContext: SubtaskItemViewModel subtask }
+            && DataContext is TaskListViewModel vm)
+        {
+            var parent = vm.Tasks.FirstOrDefault(t => t.Subtasks.Contains(subtask));
+            if (parent is not null)
+                await parent.ToggleSubtaskDoneCommand.ExecuteAsync(subtask.Id);
+        }
+    }
+
     public void FocusInlineAdd()
     {
         this.FindControl<TextBox>("InlineAddBox")?.Focus();