- useAreaAssign composable: add submitUnassignCore and loading/error states.
- DeviceDetailPage: add 'Unassign from area' button inside assign modal and
  in the actions dropdown when device has an area_id.
- ScriptDetailPage: same for scripts (non-scope) with area_id.
- AreaDetailPage: add 'Unassign from parent' in assign modal and dropdown
  when area has a parent_id > 0.