powermenu: route Sleep through suspend-then-hibernate

Aligns the third sleep path (power menu) with the lid handler and
the swayidle 30-min timer — all three now suspend immediately and
hibernate after HibernateDelaySec (30 min).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/powermenu.sh

@@ -13,6 +13,6 @@ CHOICE=$(printf "  Logout\n  Restart\n  Sleep\n  Shutdown" \
 case "$CHOICE" in
     Logout)   niri msg action quit ;;
     Restart)  systemctl reboot ;;
-    Sleep)    systemctl suspend ;;
+    Sleep)    systemctl suspend-then-hibernate ;;
     Shutdown) systemctl poweroff ;;
 esac