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.hpp

@@ -78,6 +78,7 @@ signals:
     void playlistLoaded(const QJsonObject &playlist);
     void playlistCreated(const QJsonObject &playlist);
     void playlistDeleted(const QJsonObject &result);
+    void playlistTrackAdded(qint64 playlistId);
 
     // favorites
     void favTracksLoaded(const QJsonObject &result);