feat: pagination, back/forward nav, context menu, artist fav, image fix

**Artist portrait**
- Fix CDN URL: images.portrait.{hash,format} →
  https://static.qobuz.com/images/artists/covers/large/{hash}.{format}

**Section alignment**
- Qt::ToolButtonTextOnly on all section toggles so text is truly left-aligned

**Auth 401 race condition**
- qobuz_backend_set_token now uses blocking_lock() instead of spawning an
  async task, guaranteeing the token is set before any subsequent API call

**Pagination (infinite scroll)**
- Release sections load 50 at a time (was 500)
- ArtistSection tracks has_more + loaded count; scrolling to the bottom
  emits loadMoreRequested → ArtistView calls getArtistReleases(offset=N)
- AlbumListView gains addAlbums() for append; setReleases routes to
  setAlbums (offset=0) or appendAlbums (offset>0)

**Back/Forward navigation**
- MainToolBar exposes Back/Forward QActions (go-previous/go-next icons)
- MainWindow keeps a NavPage vector + index; pushNav() on every album/artist
  navigation; goBack/goForward re-navigate without pushing history

**Context menu on now-playing label**
- Right-click on track label in toolbar → "Go to Album" / "Go to Artist"
- MainToolBar stores current track; emits albumRequested/artistRequested
  signals wired to MainWindow's existing handlers

**Artist favourites button**
- ♡ Favourite / ♥ Favourited toggle in artist header
- Calls new addFavArtist / removeFavArtist (Rust + Qt backend wiring)

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

src/backend/qobuzbackend.cpp

@@ -155,6 +155,16 @@ void QobuzBackend::removeFavAlbum(const QString &albumId)
     qobuz_backend_remove_fav_album(m_backend, albumId.toUtf8().constData());
 }
 
+void QobuzBackend::addFavArtist(qint64 artistId)
+{
+    qobuz_backend_add_fav_artist(m_backend, artistId);
+}
+
+void QobuzBackend::removeFavArtist(qint64 artistId)
+{
+    qobuz_backend_remove_fav_artist(m_backend, artistId);
+}
+
 // ---- playback ----
 
 void QobuzBackend::playTrack(qint64 trackId, int formatId)
@@ -249,7 +259,12 @@ void QobuzBackend::onEvent(int eventType, const QString &json)
         emit artistLoaded(obj);
         break;
     case 24: // EV_ARTIST_RELEASES_OK
-        emit artistReleasesLoaded(obj["release_type"].toString(), obj["items"].toArray());
+        emit artistReleasesLoaded(
+            obj["release_type"].toString(),
+            obj["items"].toArray(),
+            obj["has_more"].toBool(),
+            obj["offset"].toInt()
+        );
         break;
     case EV_ARTIST_ERR:
         emit error(obj["error"].toString());