feat: queue panel skip-to-track and drag reorder; remove visualizer

Queue panel:
- Double-clicking an upcoming track skips to it immediately: drops all
  tracks before it from the queue and starts playback (skipToUpcoming)
- Items can be dragged to reorder; rowsMoved rebuilds the queue via
  setUpcomingOrder()
- Track JSON stored per-item so order survives drag operations
- New PlayQueue methods: skipToUpcoming(), setUpcomingOrder()
- New QueuePanel signal: skipToTrackRequested(qint64) wired to MainWindow

Remove visualizer:
- Drop VisualizerWidget, Qt6::OpenGLWidgets, projectM CMake detection
- Remove qobuz_backend_read_pcm FFI (Rust + C header + Qt wrapper)
- Remove pcm_visualizer from PlayerStatus and PCM tap from AudioOutput

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/playqueue.hpp

@@ -184,6 +184,39 @@ public:
     int totalSize()      const { return m_playNext.size() + m_queue.size(); }
     int currentIndex()   const { return m_index; }
 
+    /// Skip to upcoming[upcomingIndex]: removes everything before it, pops and returns it.
+    QJsonObject skipToUpcoming(int upcomingIndex)
+    {
+        // Remove items 0..upcomingIndex-1 from the front of upcoming
+        for (int i = 0; i < upcomingIndex; ++i) {
+            if (!m_playNext.isEmpty())
+                m_playNext.removeFirst();
+            else if (m_index + 1 < m_queue.size())
+                ++m_index;
+        }
+        // Pop and return the target (now at upcoming[0])
+        if (!m_playNext.isEmpty()) {
+            const QJsonObject t = m_playNext.takeFirst();
+            emit queueChanged();
+            return t;
+        }
+        if (m_index + 1 < m_queue.size()) {
+            ++m_index;
+            emit queueChanged();
+            return m_queue.at(m_index);
+        }
+        emit queueChanged();
+        return {};
+    }
+
+    /// Replace the upcoming list with a new order (used after drag-reorder in UI).
+    void setUpcomingOrder(const QVector<QJsonObject> &newOrder)
+    {
+        m_playNext = newOrder;
+        m_queue.resize(m_index + 1); // drop old main-queue tail
+        emit queueChanged();
+    }
+
     /// Move an upcoming item (by its index in upcomingTracks()) to the front of playNext.
     void moveUpcomingToTop(int upcomingIndex)
     {