Three features wired together:
1. 'In Library' badges on explore search results:
CheckLibraryMBIDs Wails binding batch-checks which search result
MBIDs exist in the local library. Green badges render on matching
artist cards and album cards.
2. Artist images on local artist-details page:
Local artist pages now call GetArtistMBID(name) to resolve the
MBID from tags, then GetArtistImageURL(mbid) to fetch the cached
Wikimedia photo. Falls back to initial-letter avatar.
3. Tier 3 search index uses direct MBIDs from tags:
buildTier3Library now reads artists.mbid column (from audio tags)
for direct MBID matching, falling back to name matching for
untagged artists. Eliminates false matches and catches artists
that name matching misses.