Implement basic background animation for closed screen.

src/index.js

@@ -0,0 +1,9 @@
+import {startClosedAnimation} from "./closed";
+
+window.addEventListener('load', function () {
+    if (document.body.classList.contains('open')) {
+        // TODO: leave it in HTML/CSS for now.
+    } else {
+        startClosedAnimation();
+    }
+});