Change navigation layout on small screens in web UI (#34910)

app/javascript/mastodon/reducers/navigation.ts

@@ -0,0 +1,28 @@
+import { createReducer } from '@reduxjs/toolkit';
+
+import {
+  openNavigation,
+  closeNavigation,
+  toggleNavigation,
+} from 'mastodon/actions/navigation';
+
+interface State {
+  open: boolean;
+}
+
+const initialState: State = {
+  open: false,
+};
+
+export const navigationReducer = createReducer(initialState, (builder) => {
+  builder
+    .addCase(openNavigation, (state) => {
+      state.open = true;
+    })
+    .addCase(closeNavigation, (state) => {
+      state.open = false;
+    })
+    .addCase(toggleNavigation, (state) => {
+      state.open = !state.open;
+    });
+});