feat: add artist tag mode setting with split Vorbis support and improve library scan progress

- Add artist_tag_mode setting (joined / split_vorbis) for FLAC/Opus multi-artist tags
- Split 'Artist A, Artist B' into separate ARTIST= Vorbis comments when split mode is enabled
- Join repeated ARTIST/ALBUMARTIST Vorbis comments when reading metadata
- Propagate artistTagMode through download pipeline, re-enrich, and metadata editor
- Improve library scan progress: separate polling intervals, finalizing state, indeterminate progress
- Add initial progress snapshot on library scan stream connect
- Use req.ArtistName consistently for Qobuz downloads instead of track.Performer.Name
- Add l10n keys for artist tag mode, library files unit, and scan finalizing status

lib/utils/artist_utils.dart

@@ -3,6 +3,9 @@ final RegExp _artistNameSplitPattern = RegExp(
   caseSensitive: false,
 );
 
+const artistTagModeJoined = 'joined';
+const artistTagModeSplitVorbis = 'split_vorbis';
+
 List<String> splitArtistNames(String rawArtists) {
   final raw = rawArtists.trim();
   if (raw.isEmpty) return const [];
@@ -13,3 +16,25 @@ List<String> splitArtistNames(String rawArtists) {
       .where((part) => part.isNotEmpty)
       .toList(growable: false);
 }
+
+bool shouldSplitVorbisArtistTags(String mode) {
+  return mode == artistTagModeSplitVorbis;
+}
+
+List<String> splitArtistTagValues(String rawArtists) {
+  final seen = <String>{};
+  final values = <String>[];
+  for (final part in splitArtistNames(rawArtists)) {
+    final key = part.toLowerCase();
+    if (seen.add(key)) {
+      values.add(part);
+    }
+  }
+
+  if (values.isNotEmpty) {
+    return values;
+  }
+
+  final trimmed = rawArtists.trim();
+  return trimmed.isEmpty ? const [] : <String>[trimmed];
+}