Add Job.downloadProgress (0.0-1.0), derived from a worker background
poller that counts audio files landing in staging / expected track
count, since streamrip only reports 0/1. No UI yet (slice B).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
_mark_chosen ran only after a download succeeded, so throughout the 'downloading'
state no candidate was chosen — the Floor row's "source · format · tracks" line was
blank (only elapsed showed). Now mark the candidate exclusively at each download
attempt so `chosen` reflects the source in flight (and updates on fall-through).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Jonathan