Fix L7 proxy port sharing and offline messaging

[TenHopesForOneSolution]

Summary

• bind the L7 listener to the resolved public node address instead of a wildcard socket
• preserve the same external port while Docker uses the bridge as upstream
• show the custom offline kick message before anti-bot challenges when the backend is down

Validation

• GOOS=linux GOARCH=amd64 package compilation for server and l7