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/backend/qobuzbackend.hpp

@@ -31,6 +31,7 @@ public:
     void getAlbum(const QString &albumId);
     void getArtist(qint64 artistId);
     void getArtistReleases(qint64 artistId, const QString &releaseType, quint32 limit = 50, quint32 offset = 0);
+    void getAlbumsTracks(const QStringList &albumIds);
     void getPlaylist(qint64 playlistId, quint32 offset = 0, quint32 limit = 500);
 
     // --- favorites ---
@@ -88,6 +89,7 @@ signals:
     void albumLoaded(const QJsonObject &album);
     void artistLoaded(const QJsonObject &artist);
     void artistReleasesLoaded(const QString &releaseType, const QJsonArray &items, bool hasMore, int offset);
+    void deepShuffleTracksLoaded(const QJsonArray &tracks);
     void playlistLoaded(const QJsonObject &playlist);
     void playlistCreated(const QJsonObject &playlist);
     void playlistDeleted(const QJsonObject &result);