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)}`);

index.html

@@ -917,6 +917,7 @@
       <div class="info-edit-fields">
         <label class="info-edit-label">Titel<input class="info-edit-input" id="ie-title" type="text" placeholder="Titel"></label>
         <label class="info-edit-label">Künstler<input class="info-edit-input" id="ie-artist" type="text" placeholder="Künstler (mit ; trennen)"></label>
+        <label class="info-edit-label">Album-Künstler<input class="info-edit-input" id="ie-albumartist" type="text" placeholder="Album-Künstler (mit , trennen)"></label>
         <label class="info-edit-label">Album<input class="info-edit-input" id="ie-album" type="text" placeholder="Album"></label>
         <label class="info-edit-label">Genre<input class="info-edit-input" id="ie-genre" type="text" placeholder="Genre"></label>
         <div style="display:grid;grid-template-columns:1fr 1fr;gap:8px">

src/ui.js

@@ -304,6 +304,7 @@ export function toggleInfoEdit() {
   // Populate fields from current song
   document.getElementById('ie-title').value       = s.title        || '';
   document.getElementById('ie-artist').value      = s.artist       || '';
+  document.getElementById('ie-albumartist').value = s.albumArtist  || '';
   document.getElementById('ie-album').value       = s.album        || '';
   document.getElementById('ie-genre').value       = s.genre        || '';
   document.getElementById('ie-year').value        = s.year         || '';
@@ -383,6 +384,7 @@ export async function saveMetadata() {
     songId:      s.id,
     title:       document.getElementById('ie-title').value       || undefined,
     artist:      document.getElementById('ie-artist').value      || undefined,
+    albumArtist: document.getElementById('ie-albumartist').value || undefined,
     album:       document.getElementById('ie-album').value       || undefined,
     genre:       document.getElementById('ie-genre').value       || undefined,
     year:        parseInt(document.getElementById('ie-year').value)        || undefined,