Add Omarchy-style Hyprland desktop for straylight

Ports the classic dotfiles Omarchy setup to a no-home-manager NixOS
system module (configs/omarchy.nix), scoped to straylight only so the
other desktop hosts keep i3.

- Hyprland (programs.hyprland) becomes the default lightdm session; i3
  stays enabled and selectable as a fallback.
- Omarchy config layout under ~/.config/{hypr,waybar,rofi,wlogout,
  swayosd,alacritty} symlinked from the store; mutable theme/wallpaper
  state in ~/.config/omarchy/current.
- omarchy-dots generates 8 themes (hyprland/hyprlock/waybar/rofi/
  swayosd/wlogout/alacritty/mako colour files) from each theme's
  palette, with wallpapers fetched from the Omarchy v3.8.4 tag.
- Helper scripts: omarchy-theme-set/bg-next/bg-set/apply, omarchy-menu
  (rofi power/theme/background menus), waybar toggle, screenshot, lock.
- Shared desktop modules (desktop.nix / desktop-light.nix) untouched.

configs/omarchy/bin/omarchy-system-lock

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Lock the session with hyprlock, then blank the display.
+set -euo pipefail
+if ! pidof hyprlock >/dev/null; then
+  ( hyprlock; hyprctl dispatch dpms on ) &
+fi
+hyprctl switchxkblayout all 0 >/dev/null 2>&1 || true
+(
+  sleep 3
+  pidof hyprlock >/dev/null || exit 0
+  hyprctl dispatch dpms off >/dev/null 2>&1 || true
+) &
+exit 0