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/mainwindow.hpp

@@ -62,6 +62,19 @@ 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();
 };