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-toggle-waybar

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Toggle the waybar on/off (marker file read by the session autostart).
+set -euo pipefail
+CURRENT="$HOME/.config/omarchy/current"
+mkdir -p "$CURRENT"
+if [ -e "$CURRENT/waybar-off" ]; then
+  rm -f "$CURRENT/waybar-off"
+  setsid waybar >/dev/null 2>&1 &
+else
+  touch "$CURRENT/waybar-off"
+  pkill -x waybar 2>/dev/null || true
+fi
+exit 0