Add parallel_downloads config option (1 or 2) with Settings UI dropdown.
Refactor processQueue to run concurrent download slots using Promise.race,
extracting per-item logic into processOneQueueItem. Add per-item process
tracking via activeDownloads Map and cancelledItemIds Set so cancel/pause
correctly terminates all active downloads.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>