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>

README.md

@@ -190,7 +190,7 @@ examples: **[docs/OVERRIDES.md](docs/OVERRIDES.md)**.
 | `nomarchy.keyboard.variant` | `""` | XKB variant for the session |
 | `nomarchy.keyboard.devices` | `{}` | Per-device layout overrides (Hyprland `device` blocks keyed by `hyprctl devices` name) — e.g. an external keyboard with its own layout/variant |
 | `nomarchy.keyboard.layouts` | `[]` | Extra candidate layouts; when set, a watcher prompts (rofi) for a layout on a newly-connected keyboard and remembers it per-device |
-| `nomarchy.nightlight.enable` | `false` | Scheduled blue-light filter (hyprsunset) — warm at night (`.temperature`, default 4000K) between `.sunset`/`.sunrise`, no shift by day |
+| `nomarchy.nightlight.enable` | `false` | Scheduled blue-light filter (hyprsunset) — warm at night (`.temperature`, default 4000K) between `.sunset`/`.sunrise`, no shift by day; toggle it on/off from the menu (System › Night light) or the Waybar moon/sun indicator |
 | `nomarchy.hyprland.enable` | `true` | Nomarchy's Hyprland config |
 | `nomarchy.waybar.enable` | `true` | Nomarchy's Waybar |
 | `nomarchy.rofi.enable` | `true` | Themed rofi launcher + `nomarchy-menu` dispatcher |