Audio panel per-app mixer shows short system sounds too

[Mr5ol]

Small thing — the per-app mixer in the audio panel shows literally every playback stream, so a short UI sound effect (e.g. a volume-change chime) flashes into the list every time it plays. Would be nice if streams tagged media.role=event (the usual convention for notification/UI blips) got filtered out of candidateStreams in Panel.qml, similar to how omarchy_speaker_tuning streams are already excluded a few lines up.