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.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 ---