fix: use sudo to remove root-owned config backup after upgrade

Backup is created with sudo cp (owned by root), so the cleanup rm -f
fails with "Operation not permitted" when run without sudo.

https://claude.ai/code/session_01EyjEQLWbTXcZgbMDarf7NU

ubuntu-based-kiosk-v1.0.2.sh

@@ -11181,7 +11181,7 @@ upgrade_kiosk() {
     if [[ -f "$config_backup" ]]; then
         sudo cp "$config_backup" "$CONFIG_PATH"
         sudo chown "$KIOSK_USER:$KIOSK_USER" "$CONFIG_PATH"
-        rm -f "$config_backup"
+        sudo rm -f "$config_backup"
     fi
 
     # Install simplified power button handler