f5b02dd4

By: Ammar <ammar.nanjiani@gmail.com>

feat(steammachine): add Jovian NixOS host for Valve fremont hardware

Replace stock SteamOS on the Steam Machine's internal NVMe with Jovian
NixOS. Jovian's Steam Machine support is unmerged upstream (PR #584), so
this pins the author's fork branch as a separate input. Only this host
imports it; steamdeck stays on chaotic's vendored Jovian.

Host choices:

- GRUB with efiInstallAsRemovable. efibootmgr on the machine shows no
  explicit OS boot entry, only an {auto_created_boot_option} for the
  NVMe, so the firmware resolves \EFI\BOOT\BOOTX64.EFI. An NVRAM-only
  install would leave the box unable to boot.
- The host does NOT set nixpkgs.overlays. Importing the Jovian module
  already applies it via modules/jovian/overlay.nix; a second pass
  re-appends patches (pkgs/mangohud) and breaks the build.
- BIOS updates stay manual. enableFwupdBiosUpdates is on, but autoUpdate
  keeps its default of false: a BIOS flash cannot be rolled back by a
  NixOS generation.
- Theoden's game archive is mounted read-only with a host-local
  systemd.mounts block rather than modules/nixos/nfs-client.nix, which
  hardcodes its mount flags and exposes no read-only option.
- home-manager is managed by the NixOS module, so one nixos-anywhere run
  covers both system and user config.

Module change:

`gaming.desktop` (default true) gates the desktop Steam extras:
gamemode, the NixOS gamescope module, the 8BitDo udev rules,
SteamTinkerLaunch, and the launcher packages and programs (Hydra,
Heroic, Lutris, BoilR, protonup-qt, protontricks, winetricks, Vesktop,
capture tools).

Jovian already ships Steam, gamescope with cap_sys_nice, an unfiltered
`KERNEL=="hidraw*", TAG+="uaccess"` rule covering every controller, and
MangoHUD. Its performance path is gamescope plus dmemcg-booster, not
gamemode, and nothing in Jovian calls gamemoderun, so gamemode would sit
inert. The 8BitDo rules are already covered by that unfiltered rule.

So both Jovian hosts now set `desktop = false`. That also removes Hydra
from the Steam Deck, which previously inherited it unconditionally.
Save sync (Syncthing folder, Ludusavi, its timer and backup wrapper) is
unaffected and stays shared.

hardware-configuration.nix is still a placeholder. nixos-anywhere fills
it in via --generate-hardware-config during install.