feat(nightlight): menu + Waybar on/off toggle

Add a runtime toggle for the scheduled blue-light filter, the remaining
optional item on the night-light roadmap entry.

nomarchy-nightlight (nightlight.nix, always on PATH) starts/stops the
hyprsunset *service* rather than poking its runtime gamma: `systemctl
--user is-active` is the single source of truth, the screen restores to
true colours when stopped, and it lets you force the filter off for
colour-sensitive work (even at night) and back on without touching config.
It self-gates — `status` prints nothing when the unit is absent — so the
indicator hides itself when night-light isn't enabled.

- Menu: `nomarchy-menu nightlight` toggles it; a self-gated row in the
  System submenu (shown only when the hyprsunset unit exists).
- Waybar: a self-gating `custom/nightlight` indicator (moon = on, sun =
  off), in the generated bar and both summer whole-swap themes for parity.

Verified: home generation builds, generated waybar config carries
custom/nightlight, the menu has the dispatcher case + System row, both
scripts pass bash -n, and status self-gates to empty with no unit. Pending
an on-machine check that stopping hyprsunset cleanly restores the gamma.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/nightlight.nix

@@ -5,14 +5,51 @@
 #
 # The hyprsunset HM service module is provided by home-manager; this only
 # configures it. Override anything with plain services.hyprsunset.* options.
-{ config, lib, ... }:
+#
+# nomarchy-nightlight is the menu/Waybar toggle: it starts/stops the hyprsunset
+# *service* (so `systemctl --user is-active` is the single source of truth and
+# the screen restores to true colours when stopped), letting you force the
+# filter off for colour-sensitive work and back on without touching the config.
+{ config, lib, pkgs, ... }:
 
 let
   cfg = config.nomarchy.nightlight;
+
+  # Toggle/status helper, on PATH so the menu and both the generated and
+  # whole-swap Waybars can drive it by name. Self-gates: when the unit doesn't
+  # exist (night-light not enabled) `status` prints nothing, so the Waybar
+  # module hides itself — same pattern as the power-profile indicator.
+  nomarchy-nightlight = pkgs.writeShellScriptBin "nomarchy-nightlight" ''
+    unit=hyprsunset.service
+    if ! systemctl --user cat "$unit" >/dev/null 2>&1; then
+      [ "''${1:-}" = status ] && exit 0
+      notify-send "Night light" "Not enabled — set nomarchy.nightlight.enable."
+      exit 0
+    fi
+    case "''${1:-toggle}" in
+      status)
+        if systemctl --user is-active --quiet "$unit"; then
+          printf '{"text":"󰖔","tooltip":"Night light on — warm on schedule (click to disable)","class":"on"}\n'
+        else
+          printf '{"text":"󰖙","tooltip":"Night light off (click to enable)","class":"off"}\n'
+        fi ;;
+      on)  systemctl --user start "$unit" ;;
+      off) systemctl --user stop "$unit" ;;
+      toggle)
+        if systemctl --user is-active --quiet "$unit"; then
+          systemctl --user stop "$unit"
+          notify-send "Night light off" "Screen back to true colours."
+        else
+          systemctl --user start "$unit"
+          notify-send "Night light on" "Warm filter follows your schedule."
+        fi ;;
+      *) echo "usage: nomarchy-nightlight [toggle|status|on|off]" >&2; exit 64 ;;
+    esac
+  '';
 in
 {
-  config = lib.mkIf cfg.enable {
-    services.hyprsunset = {
+  config = {
+    services.hyprsunset = lib.mkIf cfg.enable {
       enable = true;
       settings.profile = [
         # Daytime: identity = no colour change.
@@ -21,5 +58,9 @@ in
         { time = cfg.sunset; temperature = cfg.temperature; }
       ];
     };
+
+    # Always on PATH (it self-gates at runtime), so a static whole-swap Waybar
+    # can exec it even when night-light is off without the call failing.
+    home.packages = [ nomarchy-nightlight ];
   };
 }