Root cause: gnome-remote-desktop-setup.service ran as root but dropped
privileges via `runuser -u gnome-remote-desktop`. Non-root grdctl --system
attempts authorisation through pkexec. The Nix-store pkexec binary is not
setuid, so every system-mode credential call silently failed while the script
still printed "configured successfully". GNOME Remote Desktop then started
without applied credentials, causing Remmina to loop at the login dialog.
Fix:
- Remove `runuser -u gnome-remote-desktop --` from grdctl_system helper;
the root-run oneshot service can call grdctl --system directly.
- Remove pkgs.polkit and pkgs.util-linux from the setup service path as
neither polkit nor runuser is needed any more.
- Update tests: rename test_setup_runs_grdctl_as_gnome_remote_desktop_user
to test_setup_runs_grdctl_directly_as_root and assert that runuser,
pkexec, and sudo are absent; add
test_privilege_escalation_packages_absent_from_setup_path.
All 21 app/tests pass.