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/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;