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/view/maincontent.hpp

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "../list/tracks.hpp"
+#include "../backend/qobuzbackend.hpp"
+
+#include <QWidget>
+#include <QLabel>
+#include <QStackedWidget>
+#include <QJsonArray>
+#include <QJsonObject>
+
+/// Central content widget — mirrors MainContent from spotify-qt.
+/// Displays either a track list, an album cover + tracks, or a search result.
+class MainContent : public QWidget
+{
+    Q_OBJECT
+
+public:
+    explicit MainContent(QobuzBackend *backend, QWidget *parent = nullptr);
+
+    List::Tracks *tracksList() const { return m_tracks; }
+
+    void showWelcome();
+    void showAlbum(const QJsonObject &album);
+    void showPlaylist(const QJsonObject &playlist);
+    void showFavTracks(const QJsonObject &result);
+    void showSearchTracks(const QJsonArray &tracks);
+
+private:
+    QobuzBackend    *m_backend = nullptr;
+    QStackedWidget  *m_stack   = nullptr;
+    QLabel          *m_welcome = nullptr;
+    List::Tracks    *m_tracks  = nullptr;
+};