MergeHelperSelectionModalViewModel now loads ParentTaskId and orders
rows hierarchy-aware (parent immediately followed by its children,
mirroring TasksIslandViewModel.Regroup). Children render indented with
the same indent-track visual as the main task list; ticking a parent
cascades selection to its children, while a child can still be
deselected on its own. A child whose parent got filtered out (e.g.
already Done) renders top-level with an explicit hint instead of
disappearing. Confirm keeps returning parents ahead of their children.
mika kuns