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/Dashboard.test.jsx

@@ -16,6 +16,7 @@ const mostPlayed = (url) => url.includes('sort=playtimeMinutes') && !url.include
 const quietRoutes = {
     'GET /api/library/summary': json(200, empty),
     'GET /api/hints': json(200, {results: []}),
+    'GET /api/news': json(200, {results: []}),
 };
 
 // Every request of the page with a Steam ID; the test of the 401 answers all but the user with 401.