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-nightlight

@@ -1,12 +1,11 @@
 #!/bin/bash
-# ABOUTME: Toggles wlsunset night light on/off.
-# ABOUTME: Kills or restarts wlsunset process, signals waybar for refresh.
+# ABOUTME: Toggles wlsunset night light on/off via systemd user service.
+# ABOUTME: Persists state across reboots (enable/disable), signals waybar for refresh.
 
-if pgrep -x wlsunset > /dev/null; then
-    pkill -x wlsunset
+if systemctl --user is-active --quiet wlsunset; then
+    systemctl --user disable --now wlsunset
 else
-    wlsunset -T 6500 -t 5000 -S 00:00 -s 00:01 &
-    disown
+    systemctl --user enable --now wlsunset
 fi
 
 pkill -RTMIN+11 waybar