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.

hosts/neogaia/default.nix

@@ -52,6 +52,8 @@
   modules.gitea-axi.enable = true;
   modules.pi.enable = true;
 
+  modules.desktop.enable = true;
+
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";
 }