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/hypr/autostart.conf

@@ -0,0 +1,14 @@
+# Autostart for the Hyprland session (Omarchy v3 autostart, adapted:
+# no uwsm, no omarchy-* first-run/hook machinery).
+
+exec-once = hypridle
+exec-once = mako
+exec-once = swayosd-server
+exec-once = clipse -listen
+exec-once = swaybg -i ~/.config/omarchy/current/background -m fill
+
+# Waybar honours `omarchy-toggle-waybar` (toggles hyprland layer autostart off)
+exec-once = sh -c 'omarchy-waybar-enabled && waybar'
+
+# Slow app launch fix -- set systemd vars
+exec-once = dbus-update-activation-environment --systemd --all