Fix logged-out web UI on smaller screens (#19263)

app/javascript/mastodon/components/not_signed_in_indicator.js

@@ -0,0 +1,12 @@
+import React from 'react';
+import { FormattedMessage } from 'react-intl';
+
+const NotSignedInIndicator = () => (
+  <div className='scrollable scrollable--flex'>
+    <div className='empty-column-indicator'>
+      <FormattedMessage id='not_signed_in_indicator.not_signed_in' defaultMessage='You need to sign in to access this resource.' />
+    </div>
+  </div>
+);
+
+export default NotSignedInIndicator;