Fix audio auto-advance and add error recovery

Summary

• Add onerror handlers to audio elements on both pages so playback skips to the next track on load failure
• Set playingFileId before calling .play() so track index is correct even if play fails
• On the random page, a failed load triggers a one-time album file re-fetch to recover from stale IDs after a re-scan
• Increase scan progress poll interval from 1s to 250ms for a more responsive progress bar