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/dialog/settings.cpp

@@ -0,0 +1,54 @@
+#include "settings.hpp"
+#include "../util/settings.hpp"
+
+#include <QVBoxLayout>
+#include <QFormLayout>
+#include <QDialogButtonBox>
+#include <QGroupBox>
+
+SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
+{
+    setWindowTitle(tr("Settings"));
+    setMinimumWidth(400);
+
+    auto *layout = new QVBoxLayout(this);
+
+    // --- Playback group ---
+    auto *playGroup  = new QGroupBox(tr("Playback"), this);
+    auto *playLayout = new QFormLayout(playGroup);
+
+    m_formatBox = new QComboBox(playGroup);
+    m_formatBox->addItem(tr("Hi-Res 24-bit/192kHz"), 27);
+    m_formatBox->addItem(tr("Hi-Res 24-bit/96kHz"),  7);
+    m_formatBox->addItem(tr("CD 16-bit"),             6);
+    m_formatBox->addItem(tr("MP3 320 kbps"),          5);
+
+    const int currentFormat = AppSettings::instance().preferredFormat();
+    for (int i = 0; i < m_formatBox->count(); ++i) {
+        if (m_formatBox->itemData(i).toInt() == currentFormat) {
+            m_formatBox->setCurrentIndex(i);
+            break;
+        }
+    }
+
+    playLayout->addRow(tr("Preferred quality:"), m_formatBox);
+    layout->addWidget(playGroup);
+
+    layout->addStretch();
+
+    auto *buttons = new QDialogButtonBox(
+        QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+    layout->addWidget(buttons);
+
+    connect(buttons, &QDialogButtonBox::accepted, this, [this] {
+        applyChanges();
+        accept();
+    });
+    connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
+}
+
+void SettingsDialog::applyChanges()
+{
+    AppSettings::instance().setPreferredFormat(
+        m_formatBox->currentData().toInt());
+}