feat(system): add gaming preset module

Opt-in `nomarchy.system.gaming.enable` (default false). Wires
`programs.steam` (with `remotePlay` and `localNetworkGameTransfers`
firewall holes opened via `mkDefault`), `programs.gamemode` (the
launching user must be in the `gamemode` group), and
`services.flatpak`.

Two pieces of the original roadmap entry split into separate
Next-column rows so the system-side preset ships now:

  1. Hyprland fullscreen-on-Steam-launch window rule (home-side).
  2. Declarative flathub remote (nixpkgs has no API for this; needs
     either an overlay or a one-shot systemd unit).

The flatpak service is enabled but the user must add flathub
manually after first boot — documented in OPTIONS.md.

core/system/gaming.nix

@@ -0,0 +1,20 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.nomarchy.system.gaming;
+in
+{
+  config = lib.mkIf cfg.enable {
+    programs.steam = {
+      enable = true;
+      remotePlay.openFirewall = lib.mkDefault true;
+      localNetworkGameTransfers.openFirewall = lib.mkDefault true;
+    };
+
+    # gamemode adjusts CPU governor and reschedules processes when a
+    # game requests it. The launching user must be in the `gamemode` group.
+    programs.gamemode.enable = true;
+
+    services.flatpak.enable = true;
+  };
+}