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.