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.

CLAUDE.md

@@ -98,3 +98,7 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
   `raichu`, a server with no desktop, is the only Nvidia machine.
   `laptop-mvi.md`'s out-of-scope line calls zeus Nvidia, but that is stale and the document is kept historical and unchanged, so do not infer any host's GPU from it.
   The corrected fact lives in ADR 0003 and the `hyprland-desktop` spec.
+- The home-manager `wayland.windowManager.hyprland` module defaults `configType` to `"lua"` at `home.stateVersion` >= 26.05, writing `hyprland.lua` through an `hl.*` Lua API instead of the native `hyprland.conf`.
+  The Lua backend mangles `$mod`-style variables and INI `bind=` strings into invalid Lua (`hl.$mod("SUPER")`), and does not fail the build, since the config is only text.
+  Set `configType = "hyprlang"` to get the native `hyprland.conf` whose variable and bind syntax the usual settings are written in.
+  Render the file to check which format is in effect: `nix build --print-out-paths .#nixosConfigurations.<host>.config.home-manager.users.<user>.xdg.configFile.\"hypr/hyprland.conf\".source` (only the enabled `configType`'s key exists).