feat: --defaults applies the default setup without the menu, also from an installer (no session): the base for a Steam Machine ISO

systemctl --user goes through user_systemctl, which only changes unit files
without a session; the theme lets Plasma build Vapor's layout at the first
login when the layout is still /etc/skel's; a one-time first-login autostart
sets up single user's launcher and opens the app.

lib/steam-game.sh

@@ -72,8 +72,8 @@ steamgame_edit() {
     fi
     "$1"; local rc=$?
     if [[ "$was_running" == true ]]; then
-        if systemctl --user is-enabled -q steam-desktop-autostart.service 2>/dev/null; then
-            systemctl --user start steam-desktop-autostart.service
+        if user_systemctl is-enabled -q steam-desktop-autostart.service 2>/dev/null; then
+            user_systemctl start steam-desktop-autostart.service
         else
             systemd-run --user --collect -q steam -silent >/dev/null 2>&1
         fi