- Rename submitAssign -> submitAssignCore in useAreaAssign composable
  to avoid name collision with local page handlers.
- Rename local page handlers -> handleAssignSubmit on Area, Device,
  and Script detail pages so event bindings call the wrapper correctly.
- Fix AppEmptyState.vue to forward #action slot into GnEmptyState's
  #actions slot so the Assign button renders for unassigned items.