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.

scripts/moonarch-doctor

@@ -178,7 +178,7 @@ section "User Services"
 if [[ $EUID -eq 0 ]]; then
     warn "Running as root — skipping user service checks"
 else
-    for svc in kanshi stasis cliphist-text cliphist-image; do
+    for svc in kanshi wlsunset stasis cliphist-text cliphist-image; do
         check_user_service "$svc"
     done
 fi