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);