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/alacritty/alacritty.toml

@@ -0,0 +1,17 @@
+# Alacritty — imports the active Omarchy theme palette.
+[general]
+import = ["~/.config/omarchy/current/theme/alacritty.toml"]
+live_config_reload = true
+
+[font]
+size = 12
+
+[font.normal]
+family = "JetBrainsMono Nerd Font"
+style = "Regular"
+
+[window]
+padding = { x = 8, y = 8 }
+
+[env]
+TERM = "xterm-256color"