fix: instant search via frontend library cache — no Go calls at all

The HTTP endpoint approach still crashed due to Wails asset server
issues. Replaced with a pure frontend solution: searchLibraryCache()
does a substring match against the libraryStore's cached artists and
albums arrays. This is pure JS — zero Go calls, zero RPC, zero
network — guaranteed instant.

Results appear immediately as the user types. The full MB+LB search
pipeline still runs via Wails RPC and replaces the library matches
with richer results when done.

Added cachedArtists/cachedAlbums getters to LibraryStore for
synchronous read-only access to the already-loaded data.
Removed the /api/search-local HTTP handler from the backend.

backend/app.go

@@ -118,9 +118,6 @@ func NewYellowJacketApp(
 		yjApp.assetHandler.RegisterHandler("/artist-images/", artistImgHandler)
 	}
 
-	// Register local search endpoint — bypasses Wails RPC serialization.
-	yjApp.assetHandler.RegisterHandler("/api/search-local", yjApp.explore.SearchLocalHandler())
-
 	// create playlist service
 	yjApp.playlist = playlist.NewService(
 		yjApp.logger, yjApp.database, yjApp.appConfig,