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/Views/MainWindow.axaml.cs

@@ -27,4 +27,14 @@ public partial class MainWindow : Window
             e.Handled = true;
         }
     }
+
+    private void OnListItemPointerPressed(object? sender, PointerPressedEventArgs e)
+    {
+        if (e.GetCurrentPoint(null).Properties.PointerUpdateKind == PointerUpdateKind.RightButtonPressed
+            && sender is Control c && c.DataContext is ListItemViewModel item
+            && DataContext is MainWindowViewModel vm)
+        {
+            vm.SelectedList = item;
+        }
+    }
 }