Add steam-close-button: X on the dashboard's Steam window

steamFrame.dashboard.steamCloseButton.enable (default false) gives the
SteamVR dashboard's main Steam window a close button: it docks the window
back into the dashboard, then switches to the previously active window or,
with none, leaves just the dashboard bar. Bar-only survives dashboard
reopens and patch restarts (state in window.__sfuiSteamCloseState, taken
over from the earlier personal version 4 of this patch); the stock
"no active frame -> show Steam" paths are suppressed while it lasts.

MobX, the dock-location enum and the closing component class are found by
signature; check-only signatures cover the dashboard internals it uses.

modules/steam-close-button.nix

@@ -0,0 +1,46 @@
+# Close (X) button on the SteamVR dashboard's main Steam window
+# (steamFrame.dashboard.steamCloseButton.enable): it switches to the
+# previously active dashboard window, or leaves just the dashboard bar when
+# Steam was the last one. Runtime patch of SteamVR's dashboard page
+# (vrwebhelper "systemui", DevTools 127.0.0.1:8087; see
+# steam-close-button/patch.js for how it works), registered with
+# steam-ui-patches.nix only when enabled; it turns on the SteamVR web helper
+# debugger (steamvr-debugger.nix), which needs one SteamVR restart the first
+# time. Depends on SteamVR UI internals, found by signature
+# (lib/signatures.json, "steam-close-button"; scripts/check-signatures.mjs
+# checks them offline).
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.steamFrame.dashboard.steamCloseButton;
+  inherit (import ./lib { inherit pkgs; }) mkPatch;
+in {
+  imports = [ ./steam-ui-patches.nix ./steamvr-debugger.nix ];
+
+  options.steamFrame.dashboard.steamCloseButton.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = false;
+    description = ''
+      Close (X) button on the SteamVR dashboard's main Steam window. It
+      docks the window back into the dashboard if it was placed in the world,
+      then switches to the most recently active other dashboard window, or,
+      with none, leaves the dashboard open with just its bar ("bar only";
+      kept when the dashboard is closed and reopened, until a window is
+      activated, e.g. with the Steam tab). Runtime patch of the SteamVR
+      dashboard (steamFrame.uiPatches), applied immediately; turning it off
+      removes the button (next switch).
+    '';
+  };
+
+  config = lib.mkIf cfg.enable {
+    steamFrame.uiPatches.patches = [ {
+      name = "steam-close-button";
+      endpoint = "http://127.0.0.1:8087";
+      target.title = "systemui";
+      patch = mkPatch {
+        name = "steam-close-button";
+        src = ./steam-close-button/patch.js;
+      };
+      unpatch = ./steam-close-button/unpatch.js;
+    } ];
+  };
+}