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/maincontent.cpp

@@ -0,0 +1,58 @@
+#include "maincontent.hpp"
+
+#include <QVBoxLayout>
+#include <QJsonArray>
+
+MainContent::MainContent(QobuzBackend *backend, QWidget *parent)
+    : QWidget(parent)
+    , m_backend(backend)
+{
+    auto *layout = new QVBoxLayout(this);
+    layout->setContentsMargins(0, 0, 0, 0);
+
+    m_stack = new QStackedWidget(this);
+    layout->addWidget(m_stack);
+
+    m_welcome = new QLabel(
+        tr("<h2>Welcome to Qobuz</h2>"
+           "<p>Select something from the library on the left to get started.</p>"),
+        this);
+    m_welcome->setAlignment(Qt::AlignCenter);
+
+    m_tracks = new List::Tracks(m_backend, this);
+
+    m_stack->addWidget(m_welcome);   // index 0
+    m_stack->addWidget(m_tracks);    // index 1
+
+    m_stack->setCurrentIndex(0);
+}
+
+void MainContent::showWelcome()
+{
+    m_stack->setCurrentIndex(0);
+}
+
+void MainContent::showAlbum(const QJsonObject &album)
+{
+    m_tracks->loadAlbum(album);
+    m_stack->setCurrentIndex(1);
+}
+
+void MainContent::showPlaylist(const QJsonObject &playlist)
+{
+    m_tracks->loadPlaylist(playlist);
+    m_stack->setCurrentIndex(1);
+}
+
+void MainContent::showFavTracks(const QJsonObject &result)
+{
+    const QJsonArray items = result["items"].toArray();
+    m_tracks->loadTracks(items);
+    m_stack->setCurrentIndex(1);
+}
+
+void MainContent::showSearchTracks(const QJsonArray &tracks)
+{
+    m_tracks->loadSearchTracks(tracks);
+    m_stack->setCurrentIndex(1);
+}