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/maintoolbar.hpp

@@ -0,0 +1,63 @@
+#pragma once
+
+#include "../backend/qobuzbackend.hpp"
+#include "../widget/volumebutton.hpp"
+#include "../widget/clickableslider.hpp"
+#include "../util/icon.hpp"
+
+#include <QToolBar>
+#include <QToolButton>
+#include <QLabel>
+#include <QAction>
+#include <QJsonObject>
+
+/// Main playback toolbar — mirrors MainToolBar from spotify-qt.
+class MainToolBar : public QToolBar
+{
+    Q_OBJECT
+
+public:
+    explicit MainToolBar(QobuzBackend *backend, QWidget *parent = nullptr);
+
+    void setPlaying(bool playing);
+    void setCurrentTrack(const QJsonObject &track);
+    void updateProgress(quint64 position, quint64 duration);
+    void setVolume(int volume);
+
+signals:
+    void searchToggled(bool visible);
+
+private slots:
+    void onPlayPause(bool checked);
+    void onPrevious();
+    void onNext();
+    void onProgressReleased();
+    void onVolumeChanged(int volume);
+
+    void onBackendStateChanged(const QString &state);
+    void onTrackChanged(const QJsonObject &track);
+    void onPositionChanged(quint64 position, quint64 duration);
+    void onTrackFinished();
+
+private:
+    QobuzBackend *m_backend = nullptr;
+
+    QAction *m_previous  = nullptr;
+    QAction *m_playPause = nullptr;
+    QAction *m_next      = nullptr;
+    QAction *m_search    = nullptr;
+
+    ClickableSlider *m_progress = nullptr;
+    QLabel          *m_timeLabel = nullptr;
+    QLabel          *m_trackLabel = nullptr;
+    VolumeButton    *m_volume = nullptr;
+
+    bool m_playing     = false;
+    bool m_seeking     = false;
+
+    // Playback queue (track IDs) for next/prev
+    QVector<qint64> m_queue;
+    int             m_queueIdx = -1;
+
+    void addSpacerWidget();
+};