chore(backend): include resolved path in delete-song "file not found" error

The delete-song endpoint returned a bare "File not found on disk" with no
context, making path-resolution mismatches impossible to diagnose. Now
reports the Navidrome path, music root, and searched filename — matching the
detail the edit-metadata endpoint already provides.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/novasonic-server.js

@@ -1341,6 +1341,7 @@ app.post('/api/delete-song', async (req, res) => {
     return res.status(502).json({ error: `Navidrome lookup failed: ${e.message}` });
   }
 
+  const ndPath = filePath; // path exactly as Navidrome reported it (for diagnostics)
   if (!path.isAbsolute(filePath)) filePath = path.join(CONFIG.musicPath, filePath);
   if (!fs.existsSync(filePath)) {
     const fileName  = path.basename(filePath);
@@ -1350,7 +1351,9 @@ app.post('/api/delete-song', async (req, res) => {
       || _findFileInDir(path.dirname(filePath), fileName)
       || _findFileByStemOrRomaji(CONFIG.musicPath, titleStem);
     if (found) filePath = found;
-    else return res.status(404).json({ error: 'File not found on disk' });
+    else return res.status(404).json({
+      error: `File not found on disk. Navidrome path: "${ndPath}" — music root: "${CONFIG.musicPath}" — searched for: "${fileName}"`,
+    });
   }
 
   // Safety: file must be inside the music root and must be an audio file