Rewrite actions/app.ts and reducers/missed_updates.ts with createAction (#24801)

app/javascript/mastodon/actions/app.ts

@@ -0,0 +1,10 @@
+import { createAction } from '@reduxjs/toolkit';
+
+export const focusApp = createAction('APP_FOCUS');
+export const unfocusApp = createAction('APP_UNFOCUS');
+
+type ChangeLayoutPayload = {
+  layout: 'mobile' | 'single-column' | 'multi-column';
+};
+export const changeLayout =
+  createAction<ChangeLayoutPayload>('APP_LAYOUT_CHANGE');