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.

docs/OPTIONS.md

@@ -87,6 +87,10 @@ Wired in `features/desktop/waybar/default.nix` (filters the battery widget out o
 
 `int`, default `32`. `XCURSOR_SIZE` when `accessibility.enable = true`. NixOS default is 24; 32 is a safer floor for low-vision users.
 
+### `nomarchy.system.gaming.enable`
+
+`bool`, default `false`. Gaming preset: enables `programs.steam` (with `remotePlay` and `localNetworkGameTransfers` firewall holes opened by `mkDefault`), `programs.gamemode` (the launching user must be in the `gamemode` group), and `services.flatpak`. The flathub remote isn't 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.
+
 ### `nomarchy.system.containers.enable`
 
 `bool`, default `false`. Rootless Podman with Docker compatibility (`docker` → `podman`), plus `podman-compose`, `podman-tui`, and `dive`.