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/xdg/niri/config.kdl

@@ -86,7 +86,7 @@ spawn-at-startup "nm-applet" "--indicator"
 spawn-sh-at-startup "waypaper --restore"
 // spawn-sh-at-startup "nemo . &> /dev/null &"
 spawn-sh-at-startup "foot --server"
-spawn-at-startup "wlsunset" "-T" "6500" "-t" "5000" "-S" "00:00" "-s" "00:01"
+// wlsunset is managed via systemd user service (wlsunset.service)
 // Clipboard history managed by cliphist.service (systemd user service)
 
 hotkey-overlay {