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>

docs/ROADMAP.md

@@ -378,10 +378,18 @@ how to override it. Items marked ✓ are shipped.
   warm at night, identity (no shift) by day. `modules/home/nightlight.nix`
   drives the HM `services.hyprsunset` with two time-based profiles
   (`sunrise` → identity, `sunset` → `temperature`), so hyprsunset handles the
-  schedule and the on-login state. Needs an on-hardware check that hyprsunset
-  applies the active profile at session start. Remaining (optional): a menu +
-  Waybar toggle to force it on/off, and geo (lat/long) auto sunset/sunrise
-  (would mean wlsunset, which schedules by location).
+  schedule and the on-login state. Active-profile-at-session-start was
+  confirmed on hardware (2026-06-18). ✓ **Menu + Waybar toggle:**
+  `nomarchy-nightlight` (in `nightlight.nix`, on PATH) starts/stops the
+  hyprsunset *service* — `systemctl --user is-active` is the single source of
+  truth, the screen restores to true colours when stopped, and it forces the
+  filter off for colour-sensitive work and back on without touching config. A
+  `nomarchy-menu nightlight` entry (System submenu, self-gated on the unit
+  existing) and a self-gating Waybar `custom/nightlight` indicator (moon = on,
+  sun = off; in the generated bar and both summer whole-swaps) drive it.
+  Remaining (optional): geo (lat/long) auto sunset/sunrise (would mean
+  wlsunset, which schedules by location). Pending an on-machine check that
+  stopping hyprsunset cleanly restores the gamma.
 - **Keyboard layouts (per-device + switching):**
   - ✓ **Per-device declarative layout:** `nomarchy.keyboard.devices`
     (`{ "<hyprctl-device-name>" = { layout; variant; }; }`) generates Hyprland