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/list/tracks.hpp

@@ -30,6 +30,7 @@ namespace List
 
         /// Set which playlist is currently displayed (0 = none).
         void setPlaylistContext(qint64 playlistId);
+        qint64 playlistId() const { return m_playlistId; }
         /// Provide the user's playlist list for the "Add to playlist" submenu.
         void setUserPlaylists(const QVector<QPair<qint64, QString>> &playlists);