Pipeline parity: redownload/start now fetches full track details from
the selected metadata source (Spotify/iTunes/Deezer) for real
track_number, disc_number, and album context. Sets explicit album
context flags so post-processing uses the standard album download path.
Stuck batch fix: active_count was 0, decremented to -1 on completion,
so batch never detected as complete. Now initializes active_count=1
and queue_index=1 since we submit the worker directly.
Button timing: Download Selected handler wired up immediately before
streaming starts, reads from window._redownloadCandidates which
updates live as results arrive. No longer blocked by slow Soulseek.
Track number: _extract_track_number_from_filename requires separator
after digits so "50 Cent" is not parsed as track 50.
Progress: real download stats from /api/downloads/status. Handles
streaming sources showing "Processing..." when no transfer found.
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