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

frontend/wailsjs/go/explore/Service.js

@@ -70,10 +70,6 @@ export function SearchLocal(arg1) {
   return window['go']['explore']['Service']['SearchLocal'](arg1);
 }
 
-export function SearchLocalHandler() {
-  return window['go']['explore']['Service']['SearchLocalHandler']();
-}
-
 export function SearchRecordings(arg1) {
   return window['go']['explore']['Service']['SearchRecordings'](arg1);
 }