When new nodes are added to a Proxmox cluster after Pulse was
initially configured, they weren't showing up in Settings. The
existing "Refresh" button only triggered network discovery, not
cluster membership re-detection.
Changes:
- Add POST /api/config/nodes/{id}/refresh-cluster endpoint
- Add "Refresh" button in cluster node panel in Settings
- Re-detect cluster membership and update stored endpoints
Related to #799