feat: add album artist field to metadata editor

Adds an editable Album-Künstler input to the info modal. Populated from
Navidrome's albumArtist field on open; saved as album_artist tag via
ffmpeg. Explicit value takes precedence; falls back to deriving from
artist (first semicolon segment) when artist is changed without an
explicit albumArtist.

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

backend/novasonic-server.js

@@ -516,7 +516,7 @@ app.get('/api/musicbrainz/lookup', async (req, res) => {
 
 // ─── METADATA EDIT ───────────────────────────────────────────────────────────
 app.post('/api/edit-metadata', async (req, res) => {
-  const { ndUrl, user, pass, songId, title, artist, album, year, trackNum, totalTracks,
+  const { ndUrl, user, pass, songId, title, artist, albumArtist, album, year, trackNum, totalTracks,
           discNum, discTotal, genre, artworkBase64, artworkMime, artworkUrl: newArtworkUrl } = req.body;
   if (!ndUrl || !user || !pass || !songId) return res.status(400).json({ error: 'ndUrl, user, pass, songId required' });
 
@@ -583,7 +583,9 @@ app.post('/api/edit-metadata', async (req, res) => {
       }
       ffArgs.push('-c', 'copy', '-map_metadata', '-1', '-id3v2_version', '3');
       if (title)       ffArgs.push('-metadata', `title=${title}`);
-      if (artist)      ffArgs.push('-metadata', `artist=${artist}`, '-metadata', `album_artist=${artist.split(';')[0].trim()}`);
+      if (artist)      ffArgs.push('-metadata', `artist=${artist}`);
+      const resolvedAlbumArtist = albumArtist || (artist ? artist.split(';')[0].trim() : null);
+      if (resolvedAlbumArtist) ffArgs.push('-metadata', `album_artist=${resolvedAlbumArtist}`);
       if (album)       ffArgs.push('-metadata', `album=${album}`);
       if (year)        ffArgs.push('-metadata', `date=${year}`);
       if (trackNum)    ffArgs.push('-metadata', `track=${totalTracks ? `${trackNum}/${totalTracks}` : String(trackNum)}`);