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/util/settings.hpp

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <QString>
+#include <QSettings>
+
+class AppSettings
+{
+public:
+    static AppSettings &instance()
+    {
+        static AppSettings inst;
+        return inst;
+    }
+
+    QString authToken() const        { return m_settings.value("auth/token").toString(); }
+    void setAuthToken(const QString &t) { m_settings.setValue("auth/token", t); }
+
+    QString userEmail() const        { return m_settings.value("auth/email").toString(); }
+    void setUserEmail(const QString &e) { m_settings.setValue("auth/email", e); }
+
+    QString displayName() const      { return m_settings.value("user/display_name").toString(); }
+    void setDisplayName(const QString &n) { m_settings.setValue("user/display_name", n); }
+
+    // 5 = MP3, 6 = CD, 7 = HiRes96, 27 = HiRes192
+    int preferredFormat() const      { return m_settings.value("playback/format", 6).toInt(); }
+    void setPreferredFormat(int f)   { m_settings.setValue("playback/format", f); }
+
+    int volume() const               { return m_settings.value("playback/volume", 80).toInt(); }
+    void setVolume(int v)            { m_settings.setValue("playback/volume", v); }
+
+    bool rememberLogin() const       { return m_settings.value("auth/remember", true).toBool(); }
+    void setRememberLogin(bool r)    { m_settings.setValue("auth/remember", r); }
+
+private:
+    AppSettings() : m_settings("qobuz-qt", "qobuz-qt") {}
+    QSettings m_settings;
+};