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/mainwindow.hpp

@@ -64,19 +64,6 @@ private:
     QDockWidget     *m_libraryDock = nullptr;
     LastFmScrobbler  *m_scrobbler   = nullptr;
 
-    // Navigation history (browser-style Back / Forward)
-    struct NavPage {
-        enum Type { None, Album, Artist } type = None;
-        QString albumId;
-        qint64  artistId = 0;
-    };
-    QVector<NavPage> m_navHistory;
-    int              m_navIndex         = -1;
-    bool             m_navFromHistory   = false;
-
     void setupMenuBar();
     void tryRestoreSession();
-    void pushNav(NavPage::Type type, const QString &albumId = {}, qint64 artistId = 0);
-    void navigateTo(const NavPage &page);
-    void updateNavButtons();
 };