fix(rdp): run grdctl --system directly as root, remove runuser/polkit/util-linux

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.

app/tests/test_rdp_module_boot_setup.py

@@ -52,9 +52,17 @@ class RdpModuleBootSetupTests(unittest.TestCase):
         self.assertIn('echo "grdctl command timed out: $*" >&2', self.setup_service)
         self.assertIn('echo "grdctl command failed (exit $rc): $*" >&2', self.setup_service)
 
-    def test_setup_runs_grdctl_as_gnome_remote_desktop_user(self):
-        self.assertIn("runuser -u gnome-remote-desktop -- \\", self.setup_service)
+    def test_setup_runs_grdctl_directly_as_root(self):
+        # The oneshot service already runs as root; system-mode grdctl must be
+        # invoked directly so it does not attempt pkexec authorisation.
         self.assertIn('grdctl --system "$@"', self.setup_service)
+        self.assertNotIn("runuser", self.setup_service)
+        self.assertNotIn("pkexec", self.setup_service)
+        self.assertNotIn("sudo", self.setup_service)
+
+    def test_privilege_escalation_packages_absent_from_setup_path(self):
+        self.assertNotIn("pkgs.polkit", self.setup_service)
+        self.assertNotIn("pkgs.util-linux", self.setup_service)
 
     def test_hub_files_are_the_source_of_truth_for_username_and_password(self):
         self.assertIn('DEFAULT_USERNAME="sovran"', self.setup_service)