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.

frontend/src/store/library-store.ts

@@ -120,6 +120,16 @@ class LibraryStore {
     // Returns cached data or fetches from backend on first access.
     // ===================================================================
 
+    /** Synchronous access to cached artists (null if not yet loaded). */
+    get cachedArtists(): library.Artist[] | null {
+        return this.artists;
+    }
+
+    /** Synchronous access to cached albums (null if not yet loaded). */
+    get cachedAlbums(): library.Album[] | null {
+        return this.albums;
+    }
+
     async getTracks(): Promise<library.Track[]> {
         if (this.tracks !== null) {
             return this.tracks;