feat(settings): remove the platform mails switch

Broadcasts go to every active user, so the settings switch and its API
call are gone. Admin broadcast texts no longer mention an opt-out.

README.md

@@ -115,7 +115,6 @@ To Be Played is split into three repositories:
   (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