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/schemas/release_groups.sql

@@ -6,6 +6,7 @@ CREATE TABLE IF NOT EXISTS release_groups (
   year                   INTEGER,
   total_tracks           INTEGER,
   total_discs            INTEGER,
+  mbid                   TEXT,
   FOREIGN KEY(cover_art_id) REFERENCES cover_art(id),
   FOREIGN KEY(album_artist_credit_id) REFERENCES artist_credit(id),
   UNIQUE(name, album_artist_credit_id)