- Add island configuration fields (island_id, island_name, is_hub, island_chain_id, hub_discovery_url, bridge_islands)
- Add NAT traversal configuration (STUN/TURN servers and credentials)
- Add DEFAULT_ISLAND_ID using UUID for new installations
- Extend PeerNode with public_address, public_port, island_id, island_chain_id, and is_hub fields
- Update DiscoveryMessage to include island metadata and public endpoint