- Revert useAreaAssign composable back to pure assign logic; remove
  submitUnassignCore and unassign state that leaked into the assign modal.
- DeviceDetailPage, ScriptDetailPage, AreaDetailPage:
  * Remove the Unassign button from the assign modal footer.
  * Add a separate GnModal confirm dialog for unassign (like Remove).
  * Dropdown action now opens the confirm dialog instead of firing
    immediately.
  * Replace non-existent ph-map-pin-slash icon with ph-x-circle.