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/explore/explore.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"log/slog"
 	"math"
-	"net/http"
 	"sort"
 	"strings"
 	"sync"
@@ -155,38 +154,6 @@ func (e *Service) SearchLocal(query string) *MBSearchResult {
 	return &result
 }
 
-// SearchLocalHandler returns an http.Handler that serves local
-// index search results as JSON.  This bypasses the Wails RPC
-// serialization queue, ensuring sub-millisecond response times.
-func (e *Service) SearchLocalHandler() http.Handler {
-	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		defer func() {
-			if rv := recover(); rv != nil {
-				e.logger.Error("search-local handler panic", "recover", rv)
-				http.Error(w, "internal error", http.StatusInternalServerError)
-			}
-		}()
-
-		query := r.URL.Query().Get("q")
-		if query == "" {
-			w.Header().Set("Content-Type", "application/json")
-			_, _ = w.Write([]byte("null"))
-			return
-		}
-
-		result := e.SearchLocal(query)
-
-		w.Header().Set("Content-Type", "application/json")
-
-		if result == nil {
-			_, _ = w.Write([]byte("null"))
-			return
-		}
-
-		_ = json.NewEncoder(w).Encode(result)
-	})
-}
-
 // ---------------------------------------------------------------------------
 // MusicBrainz lookup
 // ---------------------------------------------------------------------------