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.jsx

@@ -14,6 +14,7 @@ import DashboardWelcomeCard from '../components/DashboardWelcomeCard.jsx';
 import MostPlayedCard from '../components/MostPlayedCard.jsx';
 import PileOfShameCard from '../components/PileOfShameCard.jsx';
 import SteamGameCount from '../components/SteamGameCount';
+import WhatsNewDialog from '../components/WhatsNewDialog.jsx';
 import useDashboardData from '../hooks/useDashboardData.js';
 import useLibrarySync from '../hooks/useLibrarySync.js';
 import useForceSync from '../hooks/useForceSync.js';
@@ -130,6 +131,7 @@ const Dashboard = () => {
 
             <TileGrid tiles={DASHBOARD_TILES} titles={DASHBOARD_TITLES} layout={layout} renderTile={renderTile} editing={editing}
                       defaultLayout={DEFAULT_DASHBOARD} onSave={saveLayout} onCancel={() => setEditing(false)}/>
+            <WhatsNewDialog/>
             {/* Last child: space-y gives every other child a bottom margin, which would move the fixed card. */}
             <DashboardHint/>
         </PageContainer>