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-desktop.sh

@@ -22,18 +22,18 @@ steam_enable() {
     rm -f "$HOME/.config/autostart/steam.desktop"
 
     # Reload user systemd context and activate the background loop
-    systemctl --user daemon-reload
-    systemctl --user enable --now steam-desktop-autostart.service
+    user_systemctl daemon-reload
+    user_systemctl enable --now steam-desktop-autostart.service
 
     ok "Steam UI parameters and keyboard autostart deployed successfully."
 }
 
 steam_disable() {
     info "Removing Steam desktop autostart..."
-    systemctl --user disable --now steam-desktop-autostart.service 2>/dev/null
+    user_systemctl disable --now steam-desktop-autostart.service 2>/dev/null
     rm -f "$HOME/.config/systemd/user/steam-desktop-autostart.service" \
         "$HOME/.config/environment.d/99-kde-virtual-keyboard.conf"
-    systemctl --user daemon-reload
+    user_systemctl daemon-reload
     ok "Steam desktop settings removed (takes full effect at next login)."
 }