- Add comprehensive thread-safety with mutex locks for all shared data structures
- Implement proper tunnel cleanup when websocket connections close to prevent use-after-free
- Add immediate connection state updates when receive operations fail to prevent race conditions
- Enhance error handling with graceful failure management for SSL operations
- Prevent server crashes during client disconnections and file transfer interruptions

Root cause: Use-after-free vulnerability when freed websocket connections were still referenced by active tunnels during client interruptions.

Solution: Complete overhaul of connection lifecycle management with proper synchronization and cleanup procedures.

Fixes issue where pressing Ctrl+C during wsscp file transfers caused wssshd2 to segfault.