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/options.nix

@@ -145,6 +145,18 @@
       };
     };
 
+    gaming = {
+      enable = lib.mkEnableOption ''
+        Gaming preset: Steam (with remote-play firewall holes),
+        gamemode (CPU governor + nice on Steam launch via the user's
+        gamemode group), and flatpak. NOTE: flatpak's flathub remote
+        is not added declaratively — after first boot, run
+        `flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo`.
+        The Hyprland fullscreen-on-Steam-launch window rule is a
+        separate roadmap item.
+      '';
+    };
+
     containers = {
       enable = lib.mkEnableOption ''
         Rootless Podman with Docker compatibility (`docker` → `podman`),