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-theme-set

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Set the active Omarchy theme (name or "<theme>" from the menu).
+set -euo pipefail
+
+OMARCHY_PATH="${OMARCHY_PATH:-$HOME/.local/share/omarchy}"
+CURRENT="$HOME/.config/omarchy/current"
+
+if [ -z "${1:-}" ]; then
+  echo "Usage: omarchy-theme-set <theme-name>" >&2
+  exit 1
+fi
+
+# Normalise: strip markup, lowercase, spaces -> dashes
+name="$(printf '%s' "$1" | sed -E 's/<[^>]+>//g' | tr '[:upper:]' '[:lower:]' | tr ' ' '-')"
+
+if [ ! -d "$OMARCHY_PATH/themes/$name" ]; then
+  echo "Theme '$name' does not exist" >&2
+  exit 1
+fi
+
+printf '%s\n' "$name" >"$CURRENT/theme.name"
+# Start each theme on its first background
+find "$OMARCHY_PATH/themes/$name/backgrounds" -maxdepth 1 -type f -printf '%f\n' | sort | head -n1 \
+  >"$CURRENT/background.name"
+
+omarchy-theme-apply
+
+# Restart clients so the new colours/wallpaper take effect
+pkill -x swaybg 2>/dev/null || true
+sleep 0.1
+setsid swaybg -i "$CURRENT/background" -m fill >/dev/null 2>&1 &
+pkill -SIGUSR2 waybar 2>/dev/null || true
+hyprctl reload >/dev/null 2>&1 || true
+
+command -v notify-send >/dev/null && notify-send -u low "Theme: $name" || true
+exit 0