Slice 2 of the cover-caching pass. SyncController now downloads cover
bytes for newly-upserted albums + playlists into the shared
flutter_cache_manager disk cache after each sync transaction commits.
A cold-start scroll through the home grid paints from disk on the very
first frame instead of firing one HTTP per visible tile.
Best-effort: fire-and-forget after commit, concurrency 3, per-URL
failures swallowed (404 for collages that haven't built yet, 401
during token-refresh races). Artist covers skipped — ArtistRef.coverUrl
is server-derived from "most-recent album" and not reconstructible
client-side; album pre-warm already covers the artist's primary visual.
Auth header reuses sessionTokenProvider for parity with ServerImage.
bvandeusen
32c8d4f28f