feat: album/playlist header and playlist ownership filtering

- Add TrackContextHeader widget: shows album art (fetched via NAM),
  title, subtitle (artist/description), and metadata (year · tracks · duration)
  above the track list when an album or playlist is opened
- Hide header for favorite tracks and search results
- Store user ID in AppSettings on login
- Only show "Delete playlist" for playlists the user owns
- "Add to playlist" submenu only lists owned playlists
- "Remove from this playlist" only appears when viewing an owned playlist

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

src/list/tracks.hpp

@@ -29,7 +29,8 @@ namespace List
         void setPlayingTrackId(qint64 id);
 
         /// Set which playlist is currently displayed (0 = none).
-        void setPlaylistContext(qint64 playlistId);
+        /// isOwned controls whether "Remove from this playlist" is shown.
+        void setPlaylistContext(qint64 playlistId, bool isOwned = false);
         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);
@@ -44,6 +45,7 @@ namespace List
         QobuzBackend   *m_backend = nullptr;
         PlayQueue      *m_queue   = nullptr;
         qint64          m_playlistId = 0;
+        bool            m_playlistIsOwned = false;
         QVector<QPair<qint64, QString>> m_userPlaylists;
 
         void onDoubleClicked(const QModelIndex &index);