Adds application-level ping/pong between extension and relay server.
Extension sends PING every 15s, server echoes PONG. Round-trip time
displayed in Status tab (green/yellow/red color-coded).
Server also forwards PING with target peerId and routes PONG back,
enabling future peer-to-peer ping without server restart.
Extension already responds to incoming peer PINGs.
See CHANGELOG.md for details.
KoalaDev