Fix dunst config loading, modernize notification handling

- Point dunst to /etc/xdg/dunst/dunstrc via -conf flag (dunst with
  SYSCONFDIR=/etc does not search XDG_CONFIG_DIRS)
- Update dunstrc: fix legacy offset syntax, replace missing
  Tela-purple-dark icon theme with Colloid-Grey-Catppuccin-Dark
- Replace dunstify with notify-send in moonarch-cpugov for
  daemon-agnostic notifications, fix broken icon path
- Replace dbus-monitor based moonarch-dnd script with inline
  waybar polling (interval 2s), fixing process accumulation bug
- Add #custom-dnd to waybar CSS padding rule

defaults/xdg/niri/config.kdl

@@ -79,7 +79,7 @@ layout {
 // xwayland-satellite is managed automatically since niri 25.08
 // kanshi is managed via systemd user service (kanshi.service)
 spawn-at-startup "waybar"
-spawn-at-startup "dunst"
+spawn-at-startup "dunst" "-conf" "/etc/xdg/dunst/dunstrc"
 spawn-at-startup "/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1"
 spawn-sh-at-startup "waypaper --restore"
 // spawn-sh-at-startup "nemo . &> /dev/null &"