feat(desktop): add hyprlock and hypridle (task 0026)

Add a session-lock screen and idle management to the Hyprland-native
subdirectory, enabled through the desktop aggregator.

hyprlock draws through the compositor session-lock protocol, themed by
Stylix, with a centered field and clock. Super+X locks directly through a
guarded launch so the key works without the idle daemon.

hypridle locks on idle and powers the displays off, with tunable timeouts,
and locks before every suspend. Lid-close routes through logind to suspend,
which the pre-sleep lock covers, so the lid lands at a locked screen.

modules/desktop/hyprland/hyprlock.nix

@@ -0,0 +1,61 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# The lock screen: hyprlock, a session-lock client whose surface the compositor owns, so it survives a crash of the locker rather than exposing the session.
+let
+  cfg = config.modules.desktop.hyprlock;
+  user = config.user.name;
+
+  # The hyprlock this module installs, so the keybind and the idle daemon lock
+  # with one package and never split versions.
+  hyprlock = "${config.home-manager.users.${user}.programs.hyprlock.package}/bin/hyprlock";
+in
+{
+  options.modules.desktop.hyprlock.enable = lib.mkEnableOption "the hyprlock lock screen";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user} = {
+      # Colors and the lock-screen background come from Stylix's hyprlock target,
+      # so only geometry and behaviour are set here.
+      programs.hyprlock = {
+        enable = true;
+
+        settings = {
+          general = {
+            hide_cursor = true;
+            # No progress bar flashes before the field is ready to take input.
+            disable_loading_bar = true;
+          };
+
+          # A centered password field; its colors are the Stylix target's.
+          input-field = {
+            size = "260, 52";
+            rounding = 8;
+            position = "0, -100";
+            halign = "center";
+            valign = "center";
+          };
+
+          # The current time, above the field.
+          label = {
+            text = "$TIME";
+            font_size = 48;
+            position = "0, 120";
+            halign = "center";
+            valign = "center";
+          };
+        };
+      };
+
+      # Lock on Super+X.
+      # The guard drops the keypress when a locker is already up, so a second
+      # hyprlock never stacks over the first.
+      wayland.windowManager.hyprland.settings.bind = [
+        "$mod, X, exec, ${pkgs.procps}/bin/pidof hyprlock || ${hyprlock}"
+      ];
+    };
+  };
+}