v0.39.0

Dashboard now shows per-phone link health (contact status + round-trip time).

Each registered, idle room card now displays its SIP contact status and idle
round-trip time (e.g. "Reachable · 2.4 ms") beneath the status pill — the data was
already collected (AMI ContactList RoundtripUsec, surfaced in /api/status) but never
rendered. At a glance you can now see which handsets are responding quickly and spot
a laggy one (e.g. the Wi-Fi cordless) without opening logs. Rendering-only change to
the web dashboard; no new polling. (Trunk-registration status and whisper-server
health indicators are coming next.)