fix: resolve album-only autofill and placeholder re-enrich regressions

- Dart: _metadataMatchIsConfident now handles album-only case (title empty)
  by adding albumMatches fallback branch
- Go: selectBestReEnrichTrack treats placeholder values (Unknown Title,
  Unknown Artist) as empty via isPlaceholderReEnrichValue, so album-based
  fallback filtering works correctly
- Add test for placeholder album fallback in selectBestReEnrichTrack

lib/screens/track_metadata_edit_sheet.dart

@@ -643,6 +643,9 @@ class _EditMetadataSheetState extends State<_EditMetadataSheet> {
     if (currentTitle.isNotEmpty) {
       return titleMatches;
     }
+    if (currentAlbum.isNotEmpty) {
+      return albumMatches;
+    }
 
     return false;
   }