fix: remove gapless toggle — architecture is always gapless

The player keeps the audio output alive between tracks unconditionally.
The toggle only controlled URL prefetching, not actual audio gaplessness.
Remove the setting and always prefetch the next track URL.

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

src/mainwindow.cpp

@@ -269,8 +269,8 @@ void MainWindow::onTrackChanged(const QJsonObject &track)
     statusBar()->showMessage(
         artist.isEmpty() ? title : QStringLiteral("▶  %1 — %2").arg(artist, title));
 
-    // Gapless: prefetch next track URL so it starts immediately
-    if (AppSettings::instance().gaplessEnabled() && m_queue->canGoNext()) {
+    // Prefetch next track URL to minimise the gap between tracks
+    if (m_queue->canGoNext()) {
         const auto upcoming = m_queue->upcomingTracks(1);
         if (!upcoming.isEmpty()) {
             const qint64 nextId = static_cast<qint64>(upcoming.first()["id"].toDouble());