refactor: UI polish — lock sidebar, remove nav buttons, uniform artist tables, deep shuffle

- Lock sidebar width (setFixedWidth) so it doesn't jump between views
- Remove back/forward navigation buttons and all NavPage history code
- Uniform column layout on artist page: hide Artist column from both
  Popular Tracks and release sections, set matching fixed column widths
  so columns align vertically across all sections
- Deep shuffle: new Rust FFI endpoint fetches tracks from all albums
  in parallel, combines them, and returns via EV_DEEP_SHUFFLE_OK
- Auto-paginate artist releases in Rust (loop until has_more=false)
  so all releases load at once sorted newest-first

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

src/view/maintoolbar.hpp

@@ -24,9 +24,6 @@ public:
     void setCurrentTrack(const QJsonObject &track);
     void updateProgress(quint64 position, quint64 duration);
 
-    QAction *backAction()    const { return m_back; }
-    QAction *forwardAction() const { return m_fwd;  }
-
 signals:
     void searchToggled(bool visible);
     void queueToggled(bool visible);
@@ -57,8 +54,6 @@ private:
     QobuzBackend *m_backend = nullptr;
     PlayQueue    *m_queue   = nullptr;
 
-    QAction         *m_back        = nullptr;
-    QAction         *m_fwd         = nullptr;
     QLabel          *m_artLabel    = nullptr;
     QLabel          *m_trackLabel  = nullptr;
     QAction         *m_previous    = nullptr;