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/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); }