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/backend/qobuzbackend.hpp

@@ -0,0 +1,94 @@
+#pragma once
+
+#include "qobuz_backend.h"
+
+#include <QObject>
+#include <QString>
+#include <QJsonObject>
+#include <QTimer>
+
+/// Qt wrapper around the Rust qobuz-backend static library.
+///
+/// All signals are emitted on the Qt main thread regardless of which thread
+/// the Rust callback fires on (marshalled via QMetaObject::invokeMethod with
+/// Qt::QueuedConnection).
+class QobuzBackend : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit QobuzBackend(QObject *parent = nullptr);
+    ~QobuzBackend() override;
+
+    // --- auth ---
+    void login(const QString &email, const QString &password);
+    void setToken(const QString &token);
+
+    // --- catalog ---
+    void search(const QString &query, quint32 offset = 0, quint32 limit = 20);
+    void getAlbum(const QString &albumId);
+    void getArtist(qint64 artistId);
+    void getPlaylist(qint64 playlistId, quint32 offset = 0, quint32 limit = 500);
+
+    // --- favorites ---
+    void getFavTracks(quint32 offset = 0, quint32 limit = 500);
+    void getFavAlbums(quint32 offset = 0, quint32 limit = 200);
+    void getFavArtists(quint32 offset = 0, quint32 limit = 200);
+    void getUserPlaylists(quint32 offset = 0, quint32 limit = 200);
+
+    // --- fav modification ---
+    void addFavTrack(qint64 trackId);
+    void removeFavTrack(qint64 trackId);
+    void addFavAlbum(const QString &albumId);
+    void removeFavAlbum(const QString &albumId);
+
+    // --- playback ---
+    void playTrack(qint64 trackId, int formatId = 6);
+    void pause();
+    void resume();
+    void stop();
+    void setVolume(int volume);
+
+    quint64 position() const;
+    quint64 duration() const;
+    int     volume() const;
+    /// 1 = playing, 2 = paused, 0 = idle
+    int     state() const;
+
+signals:
+    // auth
+    void loginSuccess(const QString &token, const QJsonObject &user);
+    void loginError(const QString &error);
+
+    // catalog
+    void searchResult(const QJsonObject &result);
+    void albumLoaded(const QJsonObject &album);
+    void artistLoaded(const QJsonObject &artist);
+    void playlistLoaded(const QJsonObject &playlist);
+
+    // favorites
+    void favTracksLoaded(const QJsonObject &result);
+    void favAlbumsLoaded(const QJsonObject &result);
+    void favArtistsLoaded(const QJsonObject &result);
+    void userPlaylistsLoaded(const QJsonObject &result);
+
+    // playback
+    void trackChanged(const QJsonObject &track);
+    void stateChanged(const QString &state);
+    void positionChanged(quint64 position, quint64 duration);
+    void trackFinished();
+
+    // errors
+    void error(const QString &message);
+
+private slots:
+    Q_INVOKABLE void onEvent(int eventType, const QString &json);
+    void onPositionTick();
+
+private:
+    QobuzBackendOpaque *m_backend = nullptr;
+    QTimer *m_positionTimer = nullptr;
+
+    // Static trampoline called from Rust threads
+    static void eventTrampoline(void *userdata, int eventType, const char *json);
+};