feat(nixos): migrate system configuration

configs/nixos/modules/desktop/utilities/audio.nix

@@ -0,0 +1,19 @@
+{ config, lib, ... }:
+# PipeWire as the desktop audio server.
+let
+  cfg = config.modules.desktop.utilities.audio;
+in
+{
+  options.modules.desktop.utilities.audio.enable = lib.mkEnableOption "the PipeWire audio server";
+
+  config = lib.mkIf cfg.enable {
+    # Realtime scheduling for the audio threads, so playback survives load.
+    security.rtkit.enable = true;
+
+    services.pipewire = {
+      enable = true;
+      alsa.enable = true;
+      pulse.enable = true;
+    };
+  };
+}