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/swayosd/config.toml

@@ -0,0 +1,3 @@
+[server]
+max_volume = 100
+show_percentage = true

configs/omarchy/swayosd/style.css

@@ -0,0 +1,15 @@
+@import "../omarchy/current/theme/swayosd.css";
+
+* {
+  font-family: 'JetBrainsMono Nerd Font';
+  border-radius: 0;
+}
+
+window {
+  background-color: @background;
+  border: 2px solid @border-color;
+}
+
+label {
+  color: @label;
+}