feat(metadata): add a primary artist tag mode

Artist Tag Mode gains "Primary artist only": downloads and re-enrichment write only the first credited artist to Artist and Album Artist, so players list one artist. The FFmpeg MP3, M4A and Vorbis embeds and the AC-4 path now honor the mode; MP3 and M4A previously ignored it.

Manual metadata edits, lyrics embedding and format conversion keep existing or typed credits joined instead of dropping names. The picker scrolls so three options fit on small and landscape screens. Closes #618.

lib/models/settings.dart

@@ -26,7 +26,7 @@ class AppSettings {
   /// Zero preserves the provider's original image.
   final int embeddedCoverMaxDimension;
   final String
-  artistTagMode; // 'joined' or 'split_vorbis' for Vorbis-based formats
+  artistTagMode; // 'joined', 'split_vorbis' (Vorbis formats), or 'primary'
   final bool embedLyrics;
   final bool embedReplayGain;
   // Apply ReplayGain/R128 tags as volume normalization in the built-in player.