feat(broadcasts): add admin broadcast page and user opt-out switch

Admin page /admin/broadcasts to write, test and send broadcasts in both
languages; confirms the recipient count before send. History shows the
counts. A new switch in settings lets users opt out of platform mails;
account mails always come. Subject and body are plain text only.

README.md

@@ -107,6 +107,9 @@ To Be Played is split into three repositories:
 - **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`.
+- **Broadcast mail**: admins write a mail in both languages on `/admin/broadcasts`, send it to all active users or to
+  picked ones, send a test to themselves, confirm with the real recipient count, and see the history with the counts.
+  Users turn platform mails off in the settings; account mails always come.
 - **Welcome page**: `/welcome` opens once after the first login (Auth `settings.welcomeSeenAt` is null; only the
   dashboard redirects, deep links stay). Feature cards with links, two next steps (Steam, import) with checks from
   `/me` and the library summary. "Let's go" and "Skip" stamp it in Auth and open the dashboard. The settings page