feat(agent): v0.3.9 sync, UI details, self-update, centralized version

- Add agent/server startup sync (sync/sync_response)
- Centralize agent version in agent/VERSION + expose /api/agent/version
- Display agent version, nodeId and server version in local UI
- Add agent self-update detection/download/restart via helper scripts
- Run start/stop/delete/reset handlers in goroutines to avoid WebSocket blocking
- Update dashboard download links and SUIVI_VPN_ONDEMAND.md
- Document Podman stays installer-managed, not agent-updated

docker-compose.yml

@@ -24,6 +24,7 @@ services:
     container_name: studioe5-server
     volumes:
       - ./server/public:/app/public:ro
+      - ./agent/VERSION:/app/agent-version:ro
     restart: unless-stopped
     environment:
       DATABASE_URL: ${DATABASE_URL}