feat(sub): show the announcement on the subscription info page

The subAnnounce setting was only emitted as a base64 Announce response header, which most client apps ignore and browsers never see. Pass it into the sub page view-model and render it as an info alert at the top of the card; custom themes get the announce key for free.

Closes #5276

frontend/src/env.d.ts

@@ -18,6 +18,7 @@ interface SubPageData {
   links?: string[];
   emails?: string[];
   datepicker?: 'gregorian' | 'jalalian';
+  announce?: string;
   downloadByte?: string | number;
   uploadByte?: string | number;
   usedByte?: string | number;