Prevent two composers from being shown (#35006)

app/javascript/mastodon/hooks/useLayout.ts

@@ -0,0 +1,13 @@
+import type { LayoutType } from '../is_mobile';
+import { useAppSelector } from '../store';
+
+export const useLayout = () => {
+  const layout = useAppSelector(
+    (state) => state.meta.get('layout') as LayoutType,
+  );
+
+  return {
+    singleColumn: layout === 'single-column' || layout === 'mobile',
+    layout,
+  };
+};