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.

src/utils/settings.js

@@ -58,3 +58,11 @@ export const markWelcomeSeen = () =>
         method: 'PUT',
         signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
     });
+
+/** PUT on Auth, 204: turns the broadcast mails on or off. The caller handles the response status. */
+export const savePlatformMails = (enabled) =>
+    apiFetch(`${import.meta.env.VITE_API_URL}/api/users/settings/platform-mails`, {
+        method: 'PUT',
+        headers: {'Content-Type': 'application/json'},
+        body: JSON.stringify({enabled}),
+    });