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.

flake.nix

@@ -21,6 +21,7 @@
       launcher-menu = ./modules/launcher-menu.nix;
       steamvr-debugger = ./modules/steamvr-debugger.nix;
       dashboard-windows = ./modules/dashboard-windows.nix;
+      steam-close-button = ./modules/steam-close-button.nix;
       # Built with the consumer's pkgs; only the source comes from our input.
       # The key lets the module system deduplicate it when it is imported both
       # directly and through `default`.