fix: gapless toggle now actually controls audio output lifecycle

When gapless is off, the AudioOutput is dropped after each track ends
naturally, producing a real gap on the next play. When on, the output
stays alive so tracks transition seamlessly. Also re-adds URL prefetch
gating behind the same toggle.

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

src/mainwindow.cpp

@@ -160,6 +160,7 @@ MainWindow::MainWindow(QobuzBackend *backend, QWidget *parent)
 
     // Apply playback options from saved settings
     m_backend->setReplayGain(AppSettings::instance().replayGainEnabled());
+    m_backend->setGapless(AppSettings::instance().gaplessEnabled());
 
     tryRestoreSession();
 }
@@ -269,8 +270,8 @@ void MainWindow::onTrackChanged(const QJsonObject &track)
     statusBar()->showMessage(
         artist.isEmpty() ? title : QStringLiteral("▶  %1 — %2").arg(artist, title));
 
-    // Prefetch next track URL to minimise the gap between tracks
-    if (m_queue->canGoNext()) {
+    // Prefetch next track URL when gapless is enabled
+    if (AppSettings::instance().gaplessEnabled() && m_queue->canGoNext()) {
         const auto upcoming = m_queue->upcomingTracks(1);
         if (!upcoming.isEmpty()) {
             const qint64 nextId = static_cast<qint64>(upcoming.first()["id"].toDouble());