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/widget/volumebutton.hpp

@@ -0,0 +1,75 @@
+#pragma once
+
+#include "clickableslider.hpp"
+#include "../util/icon.hpp"
+
+#include <QToolButton>
+#include <QWidgetAction>
+#include <QMenu>
+#include <QLabel>
+#include <QVBoxLayout>
+
+/// A toolbar button that shows a volume slider popup when clicked.
+class VolumeButton : public QToolButton
+{
+    Q_OBJECT
+
+public:
+    explicit VolumeButton(QWidget *parent = nullptr) : QToolButton(parent)
+    {
+        setPopupMode(QToolButton::InstantPopup);
+        setIcon(Icon::volumeHigh());
+
+        auto *menu   = new QMenu(this);
+        auto *widget = new QWidget(menu);
+        auto *layout = new QVBoxLayout(widget);
+        layout->setContentsMargins(6, 6, 6, 6);
+
+        m_label  = new QLabel("80%", widget);
+        m_label->setAlignment(Qt::AlignCenter);
+
+        m_slider = new ClickableSlider(Qt::Vertical, widget);
+        m_slider->setRange(0, 100);
+        m_slider->setValue(80);
+        m_slider->setFixedHeight(120);
+
+        layout->addWidget(m_label);
+        layout->addWidget(m_slider);
+
+        auto *action = new QWidgetAction(menu);
+        action->setDefaultWidget(widget);
+        menu->addAction(action);
+        setMenu(menu);
+
+        connect(m_slider, &QSlider::valueChanged, this, [this](int v) {
+            m_label->setText(QString::number(v) + "%");
+            updateIcon(v);
+            emit volumeChanged(v);
+        });
+    }
+
+    int value() const { return m_slider->value(); }
+
+    void setValue(int v)
+    {
+        m_slider->blockSignals(true);
+        m_slider->setValue(v);
+        m_slider->blockSignals(false);
+        m_label->setText(QString::number(v) + "%");
+        updateIcon(v);
+    }
+
+signals:
+    void volumeChanged(int volume);
+
+private:
+    ClickableSlider *m_slider = nullptr;
+    QLabel          *m_label  = nullptr;
+
+    void updateIcon(int v)
+    {
+        if (v == 0)      setIcon(Icon::volumeMute());
+        else if (v < 50) setIcon(Icon::volumeMid());
+        else             setIcon(Icon::volumeHigh());
+    }
+};