Add Steam Deck (nix-deck) configuration with Jovian-NixOS and remote building

- Add Jovian-NixOS integration for Steam Deck hardware support
- Create nix-deck machine configuration with SteamOS role
- Add jovian-compat.nix for NixOS 25.05 compatibility (remove in 25.11+)
- Create remote-build role for distributed builds
- Configure zix790prors as build host
- Configure nix-book and nix-deck to use remote builder with fallback
- Add comprehensive setup documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

roles/desktop/default.nix

@@ -23,5 +23,6 @@ with lib;
     ./kde.nix
     ./programs.nix
     ./sddm.nix
+    ./steamos.nix
   ];
 }

roles/desktop/steamos.nix

@@ -0,0 +1,47 @@
+{ lib, config, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.roles.desktop;
+in
+{
+  options.roles.desktop.steamos = {
+    enable = mkEnableOption "SteamOS (Jovian) configuration";
+
+    autoStart = mkOption {
+      type = types.bool;
+      default = false;
+      description = "Automatically start Steam Deck UI on boot";
+    };
+
+    user = mkOption {
+      type = types.str;
+      default = "johno";
+      description = "User to run Steam as";
+    };
+
+    desktopSession = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      description = "Desktop session to launch when switching to Desktop Mode";
+    };
+
+    enableDeckyLoader = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Enable Decky Loader plugin system";
+    };
+  };
+
+  config = mkIf (cfg.enable && cfg.steamos.enable) {
+    jovian.steam = {
+      enable = true;
+      autoStart = cfg.steamos.autoStart;
+      user = cfg.steamos.user;
+      desktopSession = cfg.steamos.desktopSession;
+    };
+
+    jovian.decky-loader.enable = cfg.steamos.enableDeckyLoader;
+  };
+}