feat: 'In Library' badges, artist images on local pages, MBID-based Tier 3

Three features wired together:

1. 'In Library' badges on explore search results:
   CheckLibraryMBIDs Wails binding batch-checks which search result
   MBIDs exist in the local library. Green badges render on matching
   artist cards and album cards.

2. Artist images on local artist-details page:
   Local artist pages now call GetArtistMBID(name) to resolve the
   MBID from tags, then GetArtistImageURL(mbid) to fetch the cached
   Wikimedia photo. Falls back to initial-letter avatar.

3. Tier 3 search index uses direct MBIDs from tags:
   buildTier3Library now reads artists.mbid column (from audio tags)
   for direct MBID matching, falling back to name matching for
   untagged artists. Eliminates false matches and catches artists
   that name matching misses.

backend/explore/searchindex.go

@@ -683,8 +683,11 @@ func (si *SearchIndex) buildTier3Library(
 		}
 	}
 
-	// Read local library artist names.
-	libRows, err := si.db.QueryContext("SELECT DISTINCT name FROM artists")
+	// Read local library artists — prefer direct MBIDs from tags,
+	// fall back to name matching against the sitewide/index map.
+	libRows, err := si.db.QueryContext(
+		"SELECT DISTINCT name, mbid FROM artists",
+	)
 	if err != nil {
 		si.logger.Warn("search index: library artists query failed", "error", err)
 
@@ -693,18 +696,35 @@ func (si *SearchIndex) buildTier3Library(
 
 	defer func() { _ = libRows.Close() }()
 
-	// Collect MBIDs for matched library artists.
 	var matched []lbSitewideArtist
 
 	var resolvedMBIDs []string
 
 	for libRows.Next() {
 		var name string
-		if err := libRows.Scan(&name); err != nil {
+
+		var mbidPtr *string
+
+		if err := libRows.Scan(&name, &mbidPtr); err != nil {
 			continue
 		}
 
-		// Normalize: strip "feat." suffixes.
+		// Direct MBID from tags — most reliable.
+		if mbidPtr != nil && *mbidPtr != "" {
+			mbid := *mbidPtr
+			resolvedMBIDs = append(resolvedMBIDs, mbid)
+
+			if !indexed[mbid] {
+				matched = append(matched, lbSitewideArtist{
+					ArtistMBID: mbid,
+					ArtistName: name,
+				})
+			}
+
+			continue
+		}
+
+		// Fall back to name matching.
 		normalized := strings.ToLower(name)
 		if idx := strings.Index(normalized, " feat."); idx >= 0 {
 			normalized = normalized[:idx]
@@ -719,7 +739,6 @@ func (si *SearchIndex) buildTier3Library(
 		if a, ok := nameMap[normalized]; ok {
 			resolvedMBIDs = append(resolvedMBIDs, a.ArtistMBID)
 
-			// Only index if not already in the index from Tier 2.
 			if !indexed[a.ArtistMBID] {
 				matched = append(matched, a)
 			}