fix: play next track correctly skips to queued track

advance() was removing the playNext item then calling current(), which
fell back to m_queue[m_index] — the same track already playing. Fix:
use takeFirst() and return that item directly, only advancing m_index
when the playNext list is empty.

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

src/playqueue.hpp

@@ -124,14 +124,17 @@ public:
         return static_cast<qint64>(current()["id"].toDouble());
     }
 
-    /// Advance and return the new current track.  Returns {} at end of queue.
+    /// Advance and return the track to play next.  Returns {} at end of queue.
     QJsonObject advance()
     {
         if (!m_playNext.isEmpty()) {
-            m_playNext.removeFirst();
-        } else {
-            ++m_index;
+            // Return the playNext item directly — do NOT call current() after
+            // removal, as that would fall back to the already-playing m_index track.
+            const QJsonObject next = m_playNext.takeFirst();
+            emit queueChanged();
+            return next;
         }
+        ++m_index;
         emit queueChanged();
         return current();
     }