Rebuilds the user-settings field-builder envelope after the client applies a dynamic form action — adding or removing a field group like a temporary approver or a personal vehicle. The endpoint does not persist changes; despite being PUT, the controller action is readOnly = true. The client sends its current in-progress form state in the body and the desired action in ?action/atIndex/afterIndex; the server returns a fresh envelope with the field group added or removed and command-button indices reshuffled. Use the regular profile/settings save endpoints to commit the resulting state.