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/sqlcgen/models.go

@@ -12,6 +12,7 @@ import (
 type Artist struct {
 	ID   int64
 	Name string
+	Mbid sql.NullString
 }
 
 type ArtistCredit struct {
@@ -154,6 +155,7 @@ type ReleaseGroup struct {
 	Year                sql.NullInt64
 	TotalTracks         sql.NullInt64
 	TotalDiscs          sql.NullInt64
+	Mbid                sql.NullString
 }
 
 type ReleaseGroupRecording struct {