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/release_groups.sql

@@ -50,6 +50,7 @@ SELECT
     rg.id,
     rg.name,
     rg.year,
+    rg.mbid,
     COALESCE(ac.text, fallback_ac.text, '') as artist_name,
     COALESCE(ca.file_path, '') as cover_art_path
 FROM release_groups rg
@@ -69,6 +70,7 @@ SELECT
     rg.id,
     rg.name,
     rg.year,
+    rg.mbid,
     COALESCE(ac.text, fallback_ac.text, '') as artist_name,
     COALESCE(ca.file_path, '') as cover_art_path
 FROM release_groups rg