fix: keep SoundCloud set tracks and materialize playlists

[aimagist]

Summary

• keep SoundCloud api-v2 track URLs in real sets/playlists instead of filtering them as removed-track tombstones
• when a selected playlist contains a track that was already downloaded elsewhere, copy the existing local file into the requested playlist folder instead of silently skipping it
• show batch truth in the download UI: total tracks considered, how many were already saved, and how many are genuinely new
• add a Settings → Duplicate doctor page that previews duplicate groups and can run the existing safe library cleanup on demand
• add queue/UI regression coverage for playlist materialization, batch accounting, and the duplicate doctor entry

Verification

• npm test -- --run test/queue.test.ts test/ui.test.tsx — 67 passed
• npm run typecheck — passed
• npm run build — passed
• npm test — 388 passed