fix(soloist): apply Spotify-app volume changes to the zone (#344)

[tobsch]

Fixes #344 — root cause found and fix verified against a deterministic repro.

Diagnosis

Confirmed a bug (not by design). With Soloist, soloistPlaybackService.onEvent() never reads the volume:
updateVolume is wired only in the librespot backend (spotifyInputService.ts), and the Soloist path
forwards timing/queue/metadata but not volume.

Instrumenting onEvent and changing the app volume via the Web API shows Soloist does send it, as a
dedicated event targeted at the active device:

type=volume_changed  volume=40  zoneId=5
type=volume_changed  volume=70  zoneId=5
type=volume_changed  volume=20  zoneId=5

(The volume that also appears on every playback_state is the account-level value Connect broadcasts to
every device — zones 4/5/6/8/9 all reported volume=100 at once — so that one must not be applied, or idle
rooms follow the playing room. Only the targeted volume_changed is safe.)

Fix

Handle volume_changed in onEvent and forward to the zone, mirroring the librespot path:

if (event.type === 'volume_changed' && typeof event.volume === 'number') {
  this.controller?.updateVolume(zoneId, event.volume);
  return;
}

Verification

Web-API volume 45 → 75 → 30 on a Soloist zone (patched into the running container's dist):

• Before: the sendspin output client stays at its default; nothing changes on the speaker.
• After: the client logs Server set player volume: 45% / 75% / 30% — the app volume now reaches the
  speaker.