feat: playlist art fix + Play/Shuffle buttons in context header

- Fix playlist art: API returns images/images150/images300 arrays, not
  image_rectangle; update Rust model and Qt header to use images300 →
  images150 → images with fallback
- Add Play (▶) and Shuffle (⇄) buttons to the album/playlist header;
  Play starts from track 1, Shuffle enables shuffle mode and plays from
  a randomised position

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

src/view/maincontent.cpp

@@ -1,6 +1,7 @@
 #include "maincontent.hpp"
 
 #include <QVBoxLayout>
+#include <QPushButton>
 
 MainContent::MainContent(QobuzBackend *backend, PlayQueue *queue, QWidget *parent)
     : QWidget(parent)
@@ -31,6 +32,11 @@ MainContent::MainContent(QobuzBackend *backend, PlayQueue *queue, QWidget *paren
     tracksLayout->addWidget(m_header);
     tracksLayout->addWidget(m_tracks, 1);
 
+    QObject::connect(m_header->playButton(),    &QPushButton::clicked,
+                     [this] { m_tracks->playAll(false); });
+    QObject::connect(m_header->shuffleButton(), &QPushButton::clicked,
+                     [this] { m_tracks->playAll(true); });
+
     m_albumList  = new AlbumListView(this);
     m_artistList = new ArtistListView(this);
     m_artistView = new ArtistView(this);