Prevent assigning the last root area as a child ...

- Store: assignToArea now rejects with an error if the target area is the
  last remaining root (parent_id <= 0), preventing zero-root trees.
- UI: AreaDetailPage hides the 'Assign to area' / 'Change parent area'
  button and action-menu item when the current area is the last root,
  and shows an explanatory empty-state message instead.
- Tests: add coverage for both the rejection and the allowed case.