feat: manage wlsunset via systemd user service

Move nightlight from niri spawn-at-startup to a systemd user service
with After=kanshi.service to ensure all outputs are configured before
wlsunset starts. Toggle now uses enable/disable --now for persistent
state across reboots.

defaults/bin/moonarch-waybar-nightlight

@@ -1,8 +1,8 @@
 #!/bin/bash
 # ABOUTME: Outputs JSON status for waybar nightlight module.
-# ABOUTME: Shows warm icon when wlsunset is active, cool icon when off.
+# ABOUTME: Checks wlsunset systemd service, shows warm icon when active.
 
-if pgrep -x wlsunset > /dev/null; then
+if systemctl --user is-active --quiet wlsunset; then
     jq -nc '{text: "󰌵", alt: "on", tooltip: "Nightlight: An (5000K)", class: "on"}'
 else
     jq -nc '{text: "󰌶", alt: "off", tooltip: "Nightlight: Aus", class: "off"}'