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/rofi/config.rasi

@@ -0,0 +1,12 @@
+configuration {
+    modi: "drun,run,window";
+    show-icons: true;
+    icon-theme: "Papirus";
+    drun-display-format: "{name}";
+    display-drun: "  Apps";
+    display-run: "  Run";
+    display-window: "  Windows";
+    font: "JetBrainsMono Nerd Font 12";
+}
+
+@theme "~/.config/omarchy/current/theme/rofi.rasi"