Pages show the hall-wide notice, when there is one, at the top

The keeper's hall-announce writes one file on the shared host; the site
serves it at /hall/announcement.json, and the banner shows it with the time
in the reader's own clock, dismissable one notice at a time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Jm2auWk6RP71CjaAb4FMoG

packages/web/src/main.ts

@@ -1,4 +1,5 @@
 import { mount } from "svelte";
+import Announcement from "./Announcement.svelte";
 import * as Sentry from "@sentry/browser";
 
 // Errors only — no tracing, no replay. Without a DSN baked at build time
@@ -35,3 +36,5 @@ const page = location.pathname.startsWith("/watch/")
   ? import("./SharePage.svelte")
   : import("./App.svelte");
 page.then(({ default: Root }) => mount(Root, { target }));
+// The hall-wide notice sits above the game, apart from it: every game of Kestrel's Hall shows the same one.
+mount(Announcement, { target: document.body, anchor: document.body.firstChild ?? undefined });