feat: explicit metadata, lyrics embedding, E badge in global search

Backend:
- fetchLyrics() queries LRCLIB for plain + synced lyrics
- Synced lyrics saved as .lrc file (Navidrome reads these)
- Plain lyrics embedded via ffmpeg -metadata lyrics=
- ITUNESADVISORY=1 written for explicit tracks
- explicit flag passed through album/track download chain
- fetchItunesTracks + searchItunesAlbumTracks use country=us&explicit=Yes

Frontend:
- explicit:true sent in download payload from iTunes track data
- "E" badge shown on explicit tracks in global search track list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui.js

@@ -262,6 +262,7 @@ export async function executeDownload() {
     artworkUrl: S.downloadTarget.artworkUrl100 || S.downloadTarget.artworkUrl60 || '',
     itunesId:   S.downloadTarget.trackId || S.downloadTarget.collectionId || null,
     collectionId: S.downloadTarget.collectionId || null,
+    explicit:   S.downloadTarget.trackExplicitness === 'explicit',
   };
   addJob(jobId, payload.title, payload.artist);
   try {