feat(nightlight): menu-driven, in-flake on/off (no ~/.local/state)

Night light is off by default and configured entirely through the menu,
writing into the downstream flake instead of ~/.local/state — so the
choice is git-tracked and reproducible across a clone+rebuild.

Two flags under a new settings.* section of the state file (exposed as
the nomarchy.settings option):
- settings.nightlight.installed — sticky, gates the hyprsunset unit;
  nomarchy.nightlight.enable mkDefault-reads it. First enable from the
  menu rebuilds to create the unit (the one accepted rebuild).
- settings.nightlight.on — runtime on/off, toggled instantly
  (theme-sync set --no-switch + systemctl), no rebuild. An ExecCondition
  reads the live flag at session start so an off survives reboot;
  splitting it from the sticky flag means a later unrelated rebuild
  never undoes an instant-off (no decay).

Drops the ~/.local/state/nomarchy/nightlight-off marker and the
ConditionPathExists hack. theme-sync set now skips the wallpaper
re-apply for non-wallpaper keys, and its rebuild notifications are
generic ("changes" not "theme").

First cut of the broader principle — any user-settable config gets a
menu writer that lands it in the flake; no state outside the checkout.
Docs (README §4, ROADMAP, template) updated to reflect it.

Eval- and round-trip-verified; on-hardware check pending.

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

modules/home/rofi.nix

@@ -370,8 +370,10 @@ ${themeRows}
             && row "Audio" audio-volume-high
           command -v system-config-printer >/dev/null 2>&1 && row "Printers" printer
           row "Do Not Disturb" notification-disabled
-          systemctl --user cat hyprsunset.service >/dev/null 2>&1 \
-            && row "Night light" weather-clear-night
+          if systemctl --user is-active --quiet hyprsunset.service 2>/dev/null
+          then row "Night light (on)"  weather-clear-night
+          else row "Night light (off)" weather-clear-night
+          fi
           command -v nomarchy-snapshots >/dev/null 2>&1 && row "Snapshots" timeshift
           if [ -e "''${bats[0]}" ] && command -v powerprofilesctl >/dev/null 2>&1; then
             row "Power profile" preferences-system-power