Fixes#819
The installer was hanging at 'Pending control-plane sync detected' because
systemctl start ran synchronously, waiting for the selfheal service to complete.
If the control-plane sync failed or took a long time (e.g., Proxmox node not
configured in Pulse yet), the installer would hang indefinitely.
Changed to use 'systemctl start --no-block' to run the selfheal service
asynchronously in the background. The sync will still be attempted, but the
installer will complete immediately and show the success message.
5c6d1798a8