feat(steam): add Steam desktop module

AGENTS.md

@@ -83,3 +83,8 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
 - Pi's tool discovery checks `~/.pi/agent/bin` before `PATH`, and downloaded generic Linux binaries there can be unusable on NixOS with the stub-ld error.
   This flake patches Pi to validate local tool binaries before selecting them, so it falls back to usable `fd`/`rg` from `PATH` instead.
   Stale unpatched launchers are the remaining failure mode for broken `@` autocomplete.
+- Nix flake evaluation ignores untracked files in this checkout.
+  Keep a new auto-loaded module staged or committed until it is removed, otherwise `nix flake check` and `nixos-rebuild --flake` evaluate without it and report its options as missing.
+- The current Steam desktop client is an XWayland application.
+  Its CEF windows do not support Ozone and Steam composites them into an SDL surface with X11 extensions, so SDL Wayland selectors do not make the visible client native Wayland.
+  Keep fractional scaling sharp with Hyprland's `xwayland.force_zero_scaling` and Steam's own `STEAM_FORCE_DESKTOPUI_SCALING` instead.

hosts/neogaia/default.nix

@@ -71,6 +71,7 @@
 
   modules.desktop.enable = true;
   modules.desktop.obsidian.enable = true;
+  modules.desktop.steam.enable = true;
 
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";

modules/desktop/hyprland/hyprland.nix

@@ -112,6 +112,10 @@ in
           blur.enabled = cfg.blur;
         };
 
+        # XWayland clients render at the panel's native resolution instead of
+        # being raster-scaled by the compositor at the fractional monitor scale.
+        xwayland.force_zero_scaling = true;
+
         animations = {
           enabled = true;
           bezier = [ "ease, 0.25, 0.1, 0.25, 1.0" ];

modules/desktop/steam.nix

@@ -0,0 +1,24 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Steam game launcher and runtime integration.
+let
+  cfg = config.modules.desktop.steam;
+in
+{
+  options.modules.desktop.steam.enable = lib.mkEnableOption "Steam game launcher";
+
+  config = lib.mkIf cfg.enable {
+    programs.steam = {
+      enable = true;
+      package = pkgs.steam.override {
+        extraEnv.STEAM_FORCE_DESKTOPUI_SCALING = "1.5";
+      };
+    };
+
+    hardware.steam-hardware.enable = true;
+  };
+}

modules/desktop/waybar.nix

@@ -83,6 +83,7 @@ in
             "class<chromium.*>" = g "f268";
             "class<[Cc]ode>" = g "f121";
             "class<obsidian>" = g "f02d";
+            "class<[Ss]team>" = g "f1b6";
           };
         };