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.

template/home.nix

@@ -33,6 +33,9 @@
   #     windowMaxScale = 4.0;
   #     windowDistance.world.max = 10.0;
   #     windowDistance.theater.max = 12.0;
+  #     # X button on the Steam window: back to the previous window, or
+  #     # just the dashboard bar:
+  #     steamCloseButton.enable = true;
   #   };
   #   firefox.enable = true;             # launcher for the Firefox Flatpak
   #   # Hidden from the "+" menu (with Steam Developer Mode on):