The tab reads the v2 personalized framework (personalized_playlists), but the Discover page generates through the legacy path and nothing seeded those v2 rows -> the tab was empty. Fixes:
- New 'listening_mix' v2 generator: hands the scan's stored 'listening_recs_tracks_full' tracks to the personalized manager so the Listening Mix can mirror + Auto-Sync like every other kind (no pool hydration; can't shrink on rotation). Registered + tested.
- Sync tab now lists every registered SINGLETON kind (Listening Mix, Fresh Tape, Archives, Hidden Gems, Discovery Shuffle, Popular Picks) as a card, not just already-generated rows. Clicking 'Refresh & Mirror' runs the generator + mirrors. Variant kinds (decade/genre/daily) need a picker, so they're not auto-listed; existing variant rows still show.
Additive: new generator + frontend merge, no backend endpoint changes. End-to-end verified (refresh -> generate -> persist -> syncable tracks).
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