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/hyprland/hyprland.nix

@@ -0,0 +1,132 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# The Hyprland compositor, sourced from nixpkgs.
+let
+  cfg = config.modules.desktop.hyprland;
+  user = config.user.name;
+
+  # Numbered-workspace switch and move for 1..9, the operator's i3 muscle memory.
+  workspaceBinds = lib.concatMap (n: [
+    "$mod, ${toString n}, workspace, ${toString n}"
+    "$mod SHIFT, ${toString n}, movetoworkspace, ${toString n}"
+  ]) (lib.range 1 9);
+in
+{
+  options.modules.desktop.hyprland = {
+    enable = lib.mkEnableOption "the Hyprland compositor";
+
+    blur = lib.mkEnableOption ''
+      window blur. Off by default as the single biggest battery cost on a
+      laptop, left on for a host with the headroom to spend it'';
+  };
+
+  config = lib.mkIf cfg.enable {
+    # This program integration owns the session, portals, and polkit, launched
+    # through the universal Wayland session manager.
+    programs.hyprland = {
+      enable = true;
+      withUWSM = true;
+    };
+
+    home-manager.users.${user}.wayland.windowManager.hyprland = {
+      enable = true;
+
+      # One package drives the whole session, so there is never a version split.
+      # The program integration above installs it and the portal, leaving home-
+      # manager to write only the config.
+      package = null;
+      portalPackage = null;
+
+      # uwsm owns the systemd graphical-session targets.
+      systemd.enable = false;
+
+      # Write the native hyprlang hyprland.conf, whose variable and bind syntax
+      # the settings below are expressed in.
+      configType = "hyprlang";
+
+      settings = {
+        "$mod" = "SUPER";
+        "$terminal" = "ghostty";
+
+        input = {
+          kb_layout = "us";
+          # Caps is a second Escape.
+          # Shift+Caps still toggles a real CapsLock.
+          kb_options = "caps:escape_shifted_capslock";
+          # Snappy: a short delay before repeat begins, then a fast repeat rate.
+          repeat_delay = 250;
+          repeat_rate = 45;
+          accel_profile = "flat";
+          touchpad = {
+            natural_scroll = true;
+            tap-to-click = true;
+            disable_while_typing = true;
+          };
+        };
+
+        general = {
+          gaps_in = 4;
+          gaps_out = 8;
+          border_size = 2;
+          layout = "dwindle";
+        };
+
+        decoration = {
+          rounding = 6;
+          blur.enabled = cfg.blur;
+        };
+
+        animations = {
+          enabled = true;
+          bezier = [ "ease, 0.25, 0.1, 0.25, 1.0" ];
+          # Durations are in centiseconds.
+          # Short values keep the motion subtle.
+          animation = [
+            "windows, 1, 3, ease"
+            "fade, 1, 3, ease"
+            "workspaces, 1, 3, ease"
+            "border, 1, 3, ease"
+          ];
+        };
+
+        dwindle = {
+          pseudotile = true;
+          preserve_split = true;
+        };
+
+        bind = [
+          "$mod, Return, exec, $terminal"
+
+          # Move focus.
+          "$mod, H, movefocus, l"
+          "$mod, J, movefocus, d"
+          "$mod, K, movefocus, u"
+          "$mod, L, movefocus, r"
+
+          # Move the window within the layout.
+          "$mod SHIFT, H, movewindow, l"
+          "$mod SHIFT, J, movewindow, d"
+          "$mod SHIFT, K, movewindow, u"
+          "$mod SHIFT, L, movewindow, r"
+
+          # Resize the active window.
+          "$mod ALT, H, resizeactive, -40 0"
+          "$mod ALT, J, resizeactive, 0 40"
+          "$mod ALT, K, resizeactive, 0 -40"
+          "$mod ALT, L, resizeactive, 40 0"
+
+          "$mod, Space, togglefloating,"
+          "$mod, F, fullscreen,"
+          "$mod, T, togglesplit,"
+          "$mod SHIFT, Q, killactive,"
+          "$mod CTRL, Q, forcekillactive,"
+        ]
+        ++ workspaceBinds;
+      };
+    };
+  };
+}