Initial implementation of qobuz-qt

- Rust backend (qobuz-backend static lib): Qobuz API client (reqwest/tokio),
  Symphonia audio decoder, CPAL audio output, extern "C" FFI bridge
- Qt 6 frontend mirroring spotify-qt layout: toolbar with playback controls,
  left library dock, central track list, right search panel
- Auth: email/password login with MD5-signed requests; session token persisted
  via QSettings
- Playback: double-click a track → Rust fetches stream URL → Symphonia decodes
  → CPAL outputs to default audio device
- Dark Fusion palette matching spotify-qt feel

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

src/mainwindow.hpp

@@ -0,0 +1,52 @@
+#pragma once
+
+#include "backend/qobuzbackend.hpp"
+#include "view/maintoolbar.hpp"
+#include "view/maincontent.hpp"
+#include "view/sidepanel/view.hpp"
+#include "list/library.hpp"
+
+#include <QMainWindow>
+#include <QDockWidget>
+#include <QJsonObject>
+
+class MainWindow : public QMainWindow
+{
+    Q_OBJECT
+
+public:
+    explicit MainWindow(QobuzBackend *backend, QWidget *parent = nullptr);
+
+    static QSize defaultSize() { return {1100, 700}; }
+
+private slots:
+    void onLoginSuccess(const QString &token, const QJsonObject &user);
+    void onLoginError(const QString &error);
+
+    void onFavTracksLoaded(const QJsonObject &result);
+    void onFavAlbumsLoaded(const QJsonObject &result);
+    void onAlbumLoaded(const QJsonObject &album);
+    void onArtistLoaded(const QJsonObject &artist);
+    void onPlaylistLoaded(const QJsonObject &playlist);
+
+    void onPlayTrackRequested(qint64 trackId);
+    void onSearchAlbumSelected(const QString &albumId);
+    void onSearchArtistSelected(qint64 artistId);
+
+    void onSearchToggled(bool visible);
+
+    void showLoginDialog();
+    void showSettingsDialog();
+
+private:
+    QobuzBackend  *m_backend    = nullptr;
+    MainToolBar   *m_toolBar    = nullptr;
+    MainContent   *m_content    = nullptr;
+    List::Library *m_library    = nullptr;
+    SidePanel::View *m_sidePanel = nullptr;
+
+    QDockWidget *m_libraryDock  = nullptr;
+
+    void setupMenuBar();
+    void tryRestoreSession();
+};