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/components/WhatsNewDialog.test.jsx

@@ -0,0 +1,95 @@
+// @vitest-environment jsdom
+import {afterEach, describe, expect, it, vi} from 'vitest';
+import {act, cleanup, fireEvent, render, screen} from '@testing-library/react';
+import {toast} from 'sonner';
+import WhatsNewDialog from './WhatsNewDialog.jsx';
+import {newsText, newsTitle} from '../utils/news.js';
+import {fetchRoutes, json} from '../test/fetchRoutes.js';
+
+vi.mock('react-i18next', () => import('../test/i18n-mock.js'));
+vi.mock('sonner', () => ({toast: {error: vi.fn()}}));
+
+const NEW = {id: 2, titleEn: 'New feature', titleDe: 'Neue Funktion', textEn: 'English text', textDe: 'Deutscher Text',
+    publishedAt: '2026-10-01T10:00:00Z', seen: false};
+const ENGLISH_ONLY = {...NEW, id: 3, titleEn: 'English only', titleDe: null, textEn: 'Only English', textDe: null};
+const OLD = {...NEW, id: 1, titleEn: 'Old', titleDe: 'Alt', seen: true};
+
+const newsRoutes = (list, seen = json(204)) => fetchRoutes({'GET /api/news': list, 'PUT /api/news/seen': seen});
+const putCalls = () => fetch.mock.calls.filter(([, init]) => init?.method === 'PUT');
+
+const showNews = async (...entries) => {
+    newsRoutes(json(200, {results: entries}));
+    render(<WhatsNewDialog/>);
+    await act(async () => undefined);
+};
+
+describe('WhatsNewDialog', () => {
+    afterEach(() => {
+        cleanup();
+        vi.clearAllMocks();
+    });
+
+    it('shows only the unseen entries, German when set and English otherwise', async () => {
+        await showNews(NEW, ENGLISH_ONLY, OLD);
+
+        expect(screen.getByRole('dialog')).toBeTruthy();
+        expect(screen.getByText('Neue Funktion')).toBeTruthy();
+        expect(screen.getByText('Deutscher Text')).toBeTruthy();
+        expect(screen.getByText('English only')).toBeTruthy();
+        expect(screen.getByText('Only English')).toBeTruthy();
+        expect(screen.queryByText('Alt')).toBeNull();
+    });
+
+    it('marks the entries seen and closes on "Got it"', async () => {
+        await showNews(NEW);
+
+        fireEvent.click(screen.getByText('NEWS.GOT_IT'));
+        await act(async () => undefined);
+
+        expect(putCalls().map(([url]) => url)).toEqual([expect.stringContaining('/api/news/seen')]);
+        expect(screen.queryByRole('dialog')).toBeNull();
+    });
+
+    it('closes with X without marking anything seen', async () => {
+        await showNews(NEW);
+
+        fireEvent.click(screen.getByRole('button', {name: 'Close'}));
+
+        expect(screen.queryByRole('dialog')).toBeNull();
+        expect(putCalls()).toEqual([]);
+    });
+
+    it.each([
+        ['all entries are seen', json(200, {results: [OLD]})],
+        ['there are no entries', json(200, {results: []})],
+        ['the load fails', json(500, {})],
+    ])('shows nothing when %s', async (_, answer) => {
+        newsRoutes(answer);
+        render(<WhatsNewDialog/>);
+        await act(async () => undefined);
+
+        expect(screen.queryByRole('dialog')).toBeNull();
+        expect(toast.error).not.toHaveBeenCalled();
+    });
+
+    it('shows an error when the seen mark is not saved', async () => {
+        newsRoutes(json(200, {results: [NEW]}), json(500, {}));
+        render(<WhatsNewDialog/>);
+        await act(async () => undefined);
+
+        fireEvent.click(screen.getByText('NEWS.GOT_IT'));
+        await act(async () => undefined);
+
+        expect(toast.error).toHaveBeenCalledWith('NEWS.SAVE_FAILED');
+    });
+});
+
+describe('newsTitle / newsText', () => {
+    it('pick German only for a German UI and a set German field', () => {
+        expect(newsTitle(NEW, 'de-DE')).toBe('Neue Funktion');
+        expect(newsText(NEW, 'de')).toBe('Deutscher Text');
+        expect(newsTitle(NEW, 'en')).toBe('New feature');
+        expect(newsText(ENGLISH_ONLY, 'de')).toBe('Only English');
+        expect(newsTitle(ENGLISH_ONLY, undefined)).toBe('English only');
+    });
+});