fix: log music root in file-not-found error for diagnosis

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

backend/novasonic-server.js

@@ -769,7 +769,10 @@ app.post('/api/edit-metadata', async (req, res) => {
       || _findFileRecursive(CONFIG.musicPath, fileName)
       || (rebasedDir && _findFileInDir(rebasedDir, fileName));
     if (found) { filePath = found; }
-    else return res.status(404).json({ error: `File not found: ${filePath}` });
+    else {
+      console.warn(`[edit-metadata] File not found. nd_path=${filePath} music_root=${CONFIG.musicPath} rebased=${rebased}`);
+      return res.status(404).json({ error: `File not found: ${filePath} (music root: ${CONFIG.musicPath})` });
+    }
   }
 
   // Resolve artwork: base64 > URL