fix: graceful multi-session handling and port conflict detection

- Remove automatic lock acquisition on plugin init
- Add checkPortAvailable() function to detect if port 19222 is in use
- Start server only when port is available
- Track serverFailed state for better error messages
- browser_kill_session now waits for port to be free before starting server
- browser_status now works even without lock
- Version bump to 2.0.2