Fix audio autoplay crash from loading spinner state mismatch

Summary

• startFadeIn was setting playingFileId in an async .then() callback while loadingFileId was set synchronously, causing currentTrack to be undefined when Svelte resolved the template — triggering the "payload" error
• Set playingFileId synchronously (matching playTrack behavior), add missing onTrackStart call for scrobble tracking, and clear state on autoplay block

Test plan

• Open random page — album auto-plays first track with spinner during load
• If autoplay is browser-blocked, no spinner persists and no console error
• Click Skip — crossfade works correctly
• Manually clicking a track still works
• Build passes