feat: MBIDs in library models + local-first search + explore cache

Backend:
- Added mbid column to sqlc schemas for artists and release_groups
- Regenerated sqlc queries to SELECT mbid in artist/album queries
- Added MBID field to library.Artist and library.Album Go structs
- All GetAllArtists/GetAllAlbums variants now populate MBID

Frontend:
- Updated Wails models.ts with MBID fields on Artist and Album
- Added cachedArtists/cachedAlbums getters to LibraryStore
- searchLibraryCache now includes MBIDs and local cover art URLs
  so library results can navigate to explore detail pages
- Added mergeWithLibrary() — when full MB results arrive, library
  entries are enriched with local images and 'In Library' flags
  rather than being replaced by MB-only versions
- Created ExploreCache store for cross-page data sharing: search
  results populate the cache, detail pages can read from it to
  avoid redundant API calls for already-fetched data

backend/database/sql/queries/artists.sql

@@ -32,7 +32,7 @@ SELECT * FROM artists
 ORDER BY name;
 
 -- name: GetAlbumArtists :many
-SELECT DISTINCT a.id, a.name
+SELECT DISTINCT a.id, a.name, a.mbid
 FROM artists a
 JOIN artist_credit_artist aca ON aca.artist_id = a.id
 JOIN artist_credit ac ON ac.id = aca.credit_id
@@ -40,7 +40,7 @@ JOIN release_groups rg ON rg.album_artist_credit_id = ac.id
 ORDER BY a.name;
 
 -- name: GetAlbumArtistsByLibrary :many
-SELECT DISTINCT a.id, a.name
+SELECT DISTINCT a.id, a.name, a.mbid
 FROM artists a
 JOIN artist_credit_artist aca ON aca.artist_id = a.id
 JOIN artist_credit ac ON ac.id = aca.credit_id