feat: refresh views on all playlist mutations

- Add EV_PLAYLIST_TRACK_ADDED (22) — emitted when a track is
  successfully added to a playlist
- If the currently-open playlist is the one modified, re-fetch it
  so the track appears in the list immediately
- After creating a playlist, open it automatically so the user
  lands in the new (empty) playlist view right away
- Sidebar already refreshes on create/delete; this ensures the
  track list view also stays in sync

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

src/backend/qobuzbackend.cpp

@@ -241,6 +241,9 @@ void QobuzBackend::onEvent(int eventType, const QString &json)
     case 21: // EV_PLAYLIST_DELETED
         emit playlistDeleted(obj);
         break;
+    case 22: // EV_PLAYLIST_TRACK_ADDED
+        emit playlistTrackAdded(static_cast<qint64>(obj["playlist_id"].toDouble()));
+        break;
     case EV_GENERIC_ERR:
     case EV_TRACK_URL_ERR:
         emit error(obj["error"].toString());