replace audible bell with visual bell

Mutt rings the bell whenever the cursor moves past the last item in a list, which made routine navigation noisy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

kitty.conf

@@ -7,6 +7,10 @@ font_size 20.0
 # https://sw.kovidgoyal.net/kitty/conf.html#shortcut-kitty.Browse-scrollback-buffer-in-less
 scrollback_pager_history_size 16
 
+# https://sw.kovidgoyal.net/kitty/conf.html#opt-kitty.enable_audio_bell
+enable_audio_bell no
+# https://sw.kovidgoyal.net/kitty/conf.html#opt-kitty.visual_bell_duration
+visual_bell_duration 0.42
 # red border in i3 (termite's `urgent_on_bell`)
 window_alert_on_bell yes