- Prevents zombie TCP connections in Snapserver

- Explicit socket shutdown (shutdown(Shutdown::Both))
- Aggressive TCP keepalive (11-23s detection vs 25s)
- Network thread exit handler
- Frontend beforeunload cleanup listener

- Normal close: Instant cleanup via FIN
- App crash/kill: 11-23s cleanup via keepalive
- Network disconnect: 11-23s cleanup
- 95%+ zombie cases auto-resolve