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/backend/qobuzbackend.cpp

@@ -86,6 +86,11 @@ void QobuzBackend::setReplayGain(bool enabled)
     qobuz_backend_set_replaygain(m_backend, enabled);
 }
 
+void QobuzBackend::setGapless(bool enabled)
+{
+    qobuz_backend_set_gapless(m_backend, enabled);
+}
+
 void QobuzBackend::prefetchTrack(qint64 trackId, int formatId)
 {
     qobuz_backend_prefetch_track(m_backend, trackId, formatId);

src/backend/qobuzbackend.hpp

@@ -38,6 +38,7 @@ public:
 
     // --- playback options ---
     void setReplayGain(bool enabled);
+    void setGapless(bool enabled);
     void prefetchTrack(qint64 trackId, int formatId = 6);
 
     // --- playlist management ---

src/dialog/settings.cpp

@@ -41,6 +41,10 @@ SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
     m_replayGain->setChecked(AppSettings::instance().replayGainEnabled());
     playLayout->addRow(m_replayGain);
 
+    m_gapless = new QCheckBox(tr("Gapless playback"), playGroup);
+    m_gapless->setChecked(AppSettings::instance().gaplessEnabled());
+    playLayout->addRow(m_gapless);
+
     layout->addWidget(playGroup);
 
     // --- Last.fm group ---
@@ -96,6 +100,7 @@ void SettingsDialog::applyChanges()
 {
     AppSettings::instance().setPreferredFormat(m_formatBox->currentData().toInt());
     AppSettings::instance().setReplayGainEnabled(m_replayGain->isChecked());
+    AppSettings::instance().setGaplessEnabled(m_gapless->isChecked());
     AppSettings::instance().setLastFmEnabled(m_lastFmEnabled->isChecked());
     AppSettings::instance().setLastFmApiKey(m_lastFmApiKey->text().trimmed());
     AppSettings::instance().setLastFmApiSecret(m_lastFmApiSecret->text().trimmed());

src/dialog/settings.hpp

@@ -19,6 +19,7 @@ private:
     // Playback
     QComboBox *m_formatBox      = nullptr;
     QCheckBox *m_replayGain     = nullptr;
+    QCheckBox *m_gapless        = nullptr;
 
     // Last.fm
     QCheckBox  *m_lastFmEnabled    = nullptr;

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());

src/util/settings.hpp

@@ -38,6 +38,9 @@ public:
     bool replayGainEnabled() const     { return m_settings.value("playback/replaygain", false).toBool(); }
     void setReplayGainEnabled(bool v)  { m_settings.setValue("playback/replaygain", v); }
 
+    bool gaplessEnabled() const        { return m_settings.value("playback/gapless", false).toBool(); }
+    void setGaplessEnabled(bool v)     { m_settings.setValue("playback/gapless", v); }
+
     // --- Last.fm ---
     bool    lastFmEnabled()   const  { return m_settings.value("lastfm/enabled", false).toBool(); }
     void    setLastFmEnabled(bool v) { m_settings.setValue("lastfm/enabled", v); }