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/default.nix

@@ -22,6 +22,7 @@
     ./laptop.nix
     ./desktop.nix
     ./accessibility.nix
+    ./gaming.nix
     ./hibernate.nix
     ./containers.nix
     ./pam.nix