Previously, three layers enforced that non-admin users must have at
least one library: the backend service validation, the frontend data
provider (which silently skipped the API call for empty lists), and the
frontend form validation. This prevented both creating users without
libraries and removing all libraries from existing users. Removed the
"at least one library" constraint across all layers so that library
assignment is fully optional for non-admin users.
08b269071f