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.cpp

@@ -67,6 +67,13 @@ void QobuzBackend::getArtistReleases(qint64 artistId, const QString &releaseType
                                       releaseType.toUtf8().constData(), limit, offset);
 }
 
+void QobuzBackend::getAlbumsTracks(const QStringList &albumIds)
+{
+    const QJsonArray arr = QJsonArray::fromStringList(albumIds);
+    const QByteArray json = QJsonDocument(arr).toJson(QJsonDocument::Compact);
+    qobuz_backend_get_albums_tracks(m_backend, json.constData());
+}
+
 void QobuzBackend::getPlaylist(qint64 playlistId, quint32 offset, quint32 limit)
 {
     qobuz_backend_get_playlist(m_backend, playlistId, offset, limit);
@@ -266,6 +273,9 @@ void QobuzBackend::onEvent(int eventType, const QString &json)
             obj["offset"].toInt()
         );
         break;
+    case 25: // EV_DEEP_SHUFFLE_OK
+        emit deepShuffleTracksLoaded(obj["tracks"].toArray());
+        break;
     case EV_ARTIST_ERR:
         emit error(obj["error"].toString());
         break;