bug: restore audio on live streams and demo playback - #584

Live streams, demo playback and highlights have been silent over WebRTC since the cameras PR (#548). Clips were unaffected because they are plain mp4s, and the streamer's audio capture was fine all along.

#548 added an audio?: boolean prop to WhepPlayer, read as props.audio !== false. Vue casts an absent boolean prop to false, not undefined, so every host that does not pass it (both StreamCanvas mounts) never offered an audio transceiver and MediaMTX never sent the track. audio now defaults to true explicitly. The camera grid is unaffected, it passes :audio itself.

Also fixes a mute desync from the same PR. muted gets the same absent-to-false cast, so the player tries to unmute itself without a user gesture. When the browser blocks that, tryPlay re-muted the element but left isMuted false, so the "tap to unmute" pill never showed. They now stay in sync.

Tests: 111.