perf: batch artist image loading for library grid view

Replace per-artist sequential GetArtistMBID + GetArtistImageURL
calls (2 Wails round-trips × N artists) with a single batch
GetArtistImages(names[]) call that:

1. Resolves all names → MBIDs via AllArtistMBIDs() (one DB query)
2. Checks disk cache for each MBID via GetCachedImage (no network)
3. Returns map[name]→dataURL in one Wails bridge round-trip

Only returns already-cached images from the disk cache populated
by the index build. No network fetches triggered — artists whose
images haven't been cached yet keep the initial letter fallback
until the index build resolves them in the background.

Result: all cached artist images appear simultaneously on first
render instead of loading one-by-one over several seconds.

backend/explore/artistimage.go

@@ -76,6 +76,16 @@ func NewArtistImageProvider(
 	}
 }
 
+// GetCachedImage returns a base64 data URL from the disk cache
+// only — no network fetches.  Returns "" if not cached.
+func (p *ArtistImageProvider) GetCachedImage(artistMBID string) string {
+	if artistMBID == "" || p.imageDir == "" {
+		return ""
+	}
+
+	return p.readDiskCache(artistMBID)
+}
+
 // GetArtistImage returns a base64 data URL for the artist's photo.
 // Checks disk cache first, then resolves via MB/Wikidata and fetches
 // the image from Wikimedia Commons.  Returns "" if no image.

backend/explore/explore.go

@@ -233,6 +233,32 @@ func (e *Service) GetArtistMBID(artistName string) string {
 	return e.libMBID.GetArtistMBID(artistName)
 }
 
+// GetArtistImages resolves artist images for multiple artists by
+// name in one call.  Returns a map of artist name → base64 data
+// URL.  Only artists with cached images are returned — no network
+// fetches are triggered (use GetArtistImageURL for on-demand fetch).
+func (e *Service) GetArtistImages(names []string) map[string]string {
+	result := make(map[string]string, len(names))
+
+	// Batch resolve all names → MBIDs from the library DB.
+	allMBIDs := e.libMBID.AllArtistMBIDs()
+
+	for _, name := range names {
+		mbid, ok := allMBIDs[name]
+		if !ok || mbid == "" {
+			continue
+		}
+
+		// Only return already-cached images — don't trigger fetches.
+		img := e.artistImg.GetCachedImage(mbid)
+		if img != "" {
+			result[name] = img
+		}
+	}
+
+	return result
+}
+
 // Search concurrently queries MusicBrainz for artists, release
 // groups, and recordings matching the query, then boosts results
 // using ListenBrainz popularity data.  The final score blends