fix: preserve existing cover art when editing metadata without new image

-map 0:v? copies the embedded cover stream from the original file when
no new artwork is supplied, preventing it from being silently stripped.

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

backend/novasonic-server.js

@@ -483,7 +483,12 @@ app.post('/api/edit-metadata', async (req, res) => {
       const ffArgs = ['-i', filePath];
       if (artFile) ffArgs.push('-i', artFile);
       ffArgs.push('-map', '0:a');
-      if (artFile) ffArgs.push('-map', '1:v');
+      // Always carry cover art: new file if provided, otherwise preserve existing embedded art
+      if (artFile) {
+        ffArgs.push('-map', '1:v');
+      } else {
+        ffArgs.push('-map', '0:v?'); // '?' = optional, no error if file has no cover
+      }
       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=${artist.split(';')[0].trim()}`);
@@ -493,7 +498,7 @@ app.post('/api/edit-metadata', async (req, res) => {
       if (totalTracks) ffArgs.push('-metadata', `totaltracks=${String(totalTracks)}`);
       if (discNum)     ffArgs.push('-metadata', `disc=${discTotal ? `${discNum}/${discTotal}` : String(discNum)}`);
       if (genre)       ffArgs.push('-metadata', `genre=${genre}`);
-      if (artFile)     ffArgs.push('-metadata:s:v', 'title=Album cover', '-metadata:s:v', 'comment=Cover (front)');
+      ffArgs.push('-metadata:s:v', 'title=Album cover', '-metadata:s:v', 'comment=Cover (front)');
       ffArgs.push(tmpFile, '-y', '-loglevel', 'error');
 
       const ff = spawn('ffmpeg', ffArgs);