Set `baseURL` for pattern library

The pattern library no longer serves by default at `/ui-playground`, so
we need to set a `baseURL`.

dev-server/ui-playground/index.js

@@ -13,4 +13,4 @@ const extraRoutes = [
   },
 ];
 
-startApp({ extraRoutes, icons: sidebarIcons });
+startApp({ baseURL: '/ui-playground', extraRoutes, icons: sidebarIcons });