feat(nixos): add Signal and Vesktop to Zeus

configs/nixos/modules/desktop/apps/signal.nix

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