feat: persist APN relay secret across restarts, add dev logging and server identification to pair UI

- Electron desktop: auto-start PushRelay with secret persisted in electron-store
- CLI serve (Tauri): persist relay secret to Global.Path.state/relay-secret (mode 0600)
- Pair endpoint now returns relayURL, serverID, relaySecretHash for debugging
- Desktop settings-pair component shows server name, relay URL, and secret hash above QR
- Add console.debug logging for pairing fetch lifecycle
- Export PushRelay from node.ts entry point for Electron consumption