feat(desktop): add the desktop group and Hyprland session (task 0021)

Introduce the modules/desktop/ group behind a single modules.desktop.enable
aggregator that hand-lists and turns on each piece at default priority, so a
host enables the whole desktop with one flag yet can override any single piece.

The Hyprland compositor lives in a subdirectory of the group, sourced from
nixpkgs with the program integration owning the session, portals, and polkit
and home-manager writing the config against that one package. greetd with the
tuigreet greeter provides a mouse-free text login that launches the session
through uwsm, and Ghostty is the terminal on Super+Return.

Port the operator's KDE/i3 bindings expressed entirely in hjkl and letters,
tune input (US layout, Caps->Escape, snappy repeat, touchpad taps and natural
scroll, flat mouse accel), and set a subtle feel with blur left as a host knob.
Enable the desktop on neogaia.

modules/desktop/terminal.nix

@@ -0,0 +1,18 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Ghostty as the desktop terminal.
+let
+  cfg = config.modules.desktop.terminal;
+  user = config.user.name;
+in
+{
+  options.modules.desktop.terminal.enable = lib.mkEnableOption "Ghostty as the desktop terminal";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.home.packages = [ pkgs.ghostty ];
+  };
+}