fix: friendly error for Spotify editorial playlist 403

Backend returns {error:'EDITORIAL_PLAYLIST'} on 403 playlist routes.
Frontend shows a step-by-step workaround card instead of a red error.

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

backend/novasonic-server.js

@@ -276,6 +276,9 @@ app.post('/api/spotify/resolve', async (req, res) => {
     res.status(400).json({ error: `Unbekannter Typ: ${type}` });
   } catch(e) {
     console.error('[spotify/resolve]', e.message);
+    if (e.message.includes('403') && e.message.includes('/playlists/')) {
+      return res.status(403).json({ error: 'EDITORIAL_PLAYLIST' });
+    }
     res.status(500).json({ error: e.message });
   }
 });