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.

README.md

@@ -99,6 +99,9 @@ To Be Played is split into three repositories:
 - **Dashboard hints**: news from the admin at the bottom of the dashboard, one at a time. A "Dismiss" hint closes
   after 30 s or with X and comes back in the next browser session unless "Do not show again" is ticked; an
   "Accept" hint stays until "OK". Admins write, preview and end hints on `/admin/hints` (Auth stores them).
+- **What's new**: a dialog on the dashboard shows the news entries the user has not seen yet, newest first. "Got it"
+  marks them seen in Auth; closing the dialog any other way shows them again on the next dashboard visit. `/whats-new`
+  (menu) lists all entries and marks them seen. Admins publish, preview, edit and delete entries on `/admin/news`.
 - **Correct game data** (AUDITOR or higher): "Correct data" on the game detail page changes name, header image,
   speedrun link, moods, endless flag and genres for every user; each field goes back to the source on its own.
   "Merge duplicate" joins two entries of the same game (e.g. the Steam game and the same game added by hand) for