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/maincontent.cpp

@@ -117,3 +117,8 @@ void MainContent::setFavArtistIds(const QSet<qint64> &ids)
 {
     m_artistView->setFavArtistIds(ids);
 }
+
+void MainContent::onDeepShuffleTracks(const QJsonArray &tracks)
+{
+    m_artistView->onDeepShuffleTracks(tracks);
+}