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/waybar.nix

@@ -77,7 +77,7 @@ let
 
     modules-left = [ "hyprland/workspaces" "hyprland/window" ];
     modules-center = [ "clock" ];
-    modules-right = [ "tray" "pulseaudio" "cpu" "memory" "custom/powerprofile" ]
+    modules-right = [ "tray" "pulseaudio" "cpu" "memory" "custom/powerprofile" "custom/nightlight" ]
       ++ lib.optional showLanguage "hyprland/language"
       ++ [ "battery" "custom/notification" ];
 
@@ -131,6 +131,16 @@ let
       on-click = "nomarchy-powerprofile-cycle";
     };
 
+    # Night-light (hyprsunset) toggle + state. Self-gates: hidden unless
+    # nomarchy.nightlight is enabled (the status helper prints nothing then).
+    # Click starts/stops the schedule; moon = on, sun = off.
+    "custom/nightlight" = {
+      exec = "nomarchy-nightlight status";
+      return-type = "json";
+      interval = 3;
+      on-click = "nomarchy-nightlight toggle";
+    };
+
     # swaync notification bell + Do-Not-Disturb state. `-swb` streams JSON
     # (text/tooltip/class) on every change, so it tracks count and DND with
     # no polling. Left-click toggles the panel; right-click toggles DND.
@@ -202,11 +212,14 @@ let
       font-weight: bold;
     }
 
-    #tray, #pulseaudio, #cpu, #memory, #custom-powerprofile, #language, #battery, #custom-notification {
+    #tray, #pulseaudio, #cpu, #memory, #custom-powerprofile, #custom-nightlight, #language, #battery, #custom-notification {
       color: @subtext;
       padding: 0 10px;
     }
 
+    /* Night-light active → warm tone, matching the filter it represents. */
+    #custom-nightlight.on { color: @warn; }
+
     /* The power-profile speedometer glyph renders small in its em box —
        size it up so it reads at a glance like the other indicators. */
     #custom-powerprofile { font-size: ${toString (t.fonts.size + 3)}pt; }