fix(dnd): add the notification bell to the whole-swap summer themes

The DND bell only landed in the generated Waybar config; summer-day and
summer-night ship waybar.jsonc whole-swaps that replace it entirely, so
they'd have shown no bell. Add custom/notification (swaync-client -swb,
left-click panel / right-click DND) to both legacy layouts with their
FontAwesome bell / bell-slash glyphs, and style it alongside the other
right-side modules in each waybar.css.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

themes/summer-night/waybar.css

@@ -56,6 +56,7 @@ window#waybar {
 #network,
 #idle_inhibitor,
 #battery,
+#custom-notification,
 #custom-powermenu {
   background-color: @bg0;
   color: @fg;