fix: remove gapless toggle — architecture is always gapless

The player keeps the audio output alive between tracks unconditionally.
The toggle only controlled URL prefetching, not actual audio gaplessness.
Remove the setting and always prefetch the next track URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/dialog/settings.hpp

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