fix(ui): context menu operates on right-clicked item and gates new-task on list selection

- Add PointerPressed handlers on list/task item templates that set SelectedList/SelectedTask on right-click before the ContextFlyout opens
- Add CanAddTask guard and NotifyCanExecuteChangedFor on CurrentListId so Add Task menu item is disabled when no list is selected

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

src/ClaudeDo.Ui/ViewModels/TaskListViewModel.cs

@@ -24,7 +24,7 @@ public partial class TaskListViewModel : ViewModelBase
     public ObservableCollection<TaskItemViewModel> Tasks { get; } = new();
 
     [ObservableProperty] private TaskItemViewModel? _selectedTask;
-    [ObservableProperty] private string? _currentListId;
+    [ObservableProperty, NotifyCanExecuteChangedFor(nameof(AddTaskCommand))] private string? _currentListId;
 
     public event Action<TaskItemViewModel?>? SelectedTaskChanged;
 
@@ -69,7 +69,9 @@ public partial class TaskListViewModel : ViewModelBase
         }
     }
 
-    [RelayCommand]
+    private bool CanAddTask() => CurrentListId is not null;
+
+    [RelayCommand(CanExecute = nameof(CanAddTask))]
     private async Task AddTask()
     {
         if (CurrentListId is null) return;