test: data router, one render helper, fetchRoutes in every test

src/pages/PlayNextPage.language.test.jsx

@@ -3,6 +3,7 @@ import {afterEach, beforeEach, describe, expect, it, vi} from 'vitest';
 import {cleanup, render, screen, waitFor} from '@testing-library/react';
 import {MemoryRouter} from 'react-router-dom';
 import PlayNextPage from './PlayNextPage.jsx';
+import {fetchRoutes, json} from '../test/fetchRoutes.js';
 import {PreferencesProvider} from '../context/PreferencesContext.jsx';
 
 // react-i18next hands out a new `t` on every language change; this mock does the same on demand.
@@ -19,7 +20,7 @@ describe('PlayNextPage on a language change', () => {
     beforeEach(() => {
         localStorage.setItem('tbp_session', '1');
         i18n.t = (key) => key;
-        globalThis.fetch = vi.fn(async () => ({ok: true, status: 200, json: async () => ({games: [game(1)]})}));
+        fetchRoutes({'GET /api/library/picker': json(200, {games: [game(1)]})});
     });
 
     afterEach(() => {