CoverArtProxy now checks three sources in order:
1. Local library (instant) — matches by album+artist name against
the release_groups/cover_art tables. Albums the user already
owns show their local cover art immediately.
2. Disk cache (instant) — previously fetched CAA thumbnails.
3. Cover Art Archive (network) — fetches and caches to disk.
Library index is built once on first access (sync.Once) from a
single SQL query joining release_groups → cover_art → artists.
Keyed by lowercased 'album\x00artist' for exact name matching.
GetThumbnail now takes (mbid, albumName, artistName) so the proxy
can check the library before falling back to CAA. Frontend passes
the album title and artist credit from the search result.
Add CoverArtProxy that fetches cover art from CAA, caches the image
bytes on disk (~/.local/share/yellowjacket/cover-art-cache/), and
returns base64 data URLs via the GetThumbnail Wails binding.
First load: fetches from CAA (rate-limited), caches to disk.
Subsequent loads: instant from disk cache, no network.
404s: cached as empty files to avoid re-fetching.
Frontend explore-view loads thumbnails async via GetThumbnail()
calls that fire during render. Cached thumbnails appear as data
URLs directly in img src, bypassing the browser's HTTP stack.
Uncached thumbnails fall back to the CAA URL while the proxy
fetches in the background, then re-render with the cached version.
Also stores caa_id and caa_release_mbid in the search index's
extra_json for future direct Internet Archive URL construction.
yonlu