fix: SC metadata fields, albumartist & separator, importer clear, folder rename on album edit

- SC tracks: use artworkUrl/album/genre fields (not iTunes artworkUrl100/collectionName/primaryGenreName)
- Playlist handler: same SC field fallbacks for artwork, album, genre, year
- ytdlp.js: convert & to , in album_artist tag so Navidrome albumartist.Split works
- Importer: clear both previews on any URL input change, not just on clear
- edit-metadata: rename album folder on disk when album name changes

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

backend/lib/ytdlp.js

@@ -312,7 +312,7 @@ async function embedMetadata({ jobId, audioFile, dir, trackPrefix, safeTitle, ar
       '-id3v2_version', '3',
       '-metadata', `title=${title}`,
       '-metadata', `artist=${navidromeArtist(artist)}`,
-      '-metadata', `album_artist=${mainArtist}`,
+      '-metadata', `album_artist=${mainArtist.replace(/\s+&\s+/g, ', ')}`,
     );
     if (album)       ffArgs.push('-metadata', `album=${album}`);
     if (year)        ffArgs.push('-metadata', `date=${String(year)}`);