feat: add What's new feature

Dashboard dialog shows unseen news entries. Users mark them seen with "Got it"
or revisit via the /whats-new page. Admins publish, edit, and delete entries
on /admin/news. Full i18n support and test coverage included.

src/pages/WhatsNewPage.test.jsx

@@ -0,0 +1,45 @@
+// @vitest-environment jsdom
+import {afterEach, describe, expect, it, vi} from 'vitest';
+import {cleanup, screen} from '@testing-library/react';
+import WhatsNewPage from './WhatsNewPage.jsx';
+import {fetchRoutes, json} from '../test/fetchRoutes.js';
+import {renderWithProviders} from '../test/renderWithProviders.jsx';
+
+vi.mock('react-i18next', () => import('../test/i18n-mock.js'));
+
+const entry = (id, title) => ({id, titleEn: title, titleDe: null, textEn: `${title} text`, textDe: null,
+    publishedAt: '2026-10-01T10:00:00Z', seen: false});
+const putCalls = () => fetch.mock.calls.filter(([, init]) => init?.method === 'PUT');
+
+describe('WhatsNewPage', () => {
+    afterEach(() => {
+        cleanup();
+        vi.clearAllMocks();
+    });
+
+    it('lists the entries in the given order and marks them seen once', async () => {
+        fetchRoutes({'GET /api/news': json(200, {results: [entry(2, 'Second'), entry(1, 'First')]}),
+            'PUT /api/news/seen': json(204)});
+        renderWithProviders(<WhatsNewPage/>);
+
+        await screen.findByText('Second');
+
+        expect(screen.getAllByRole('heading', {level: 3}).map((heading) => heading.textContent)).toEqual(['Second', 'First']);
+        expect(putCalls()).toHaveLength(1);
+    });
+
+    it('says so when there are no entries', async () => {
+        fetchRoutes({'GET /api/news': json(200, {results: []}), 'PUT /api/news/seen': json(204)});
+        renderWithProviders(<WhatsNewPage/>);
+
+        expect(await screen.findByText('NEWS.EMPTY')).toBeTruthy();
+    });
+
+    it('shows an error and marks nothing when the load fails', async () => {
+        fetchRoutes({'GET /api/news': json(500, {})});
+        renderWithProviders(<WhatsNewPage/>);
+
+        expect(await screen.findByText('NEWS.LOAD_ERROR')).toBeTruthy();
+        expect(putCalls()).toEqual([]);
+    });
+});