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

src/routes/+layout.svelte

@@ -1,7 +1,9 @@
 <script lang="ts">
 	import '../app.css';
+	import Announcement from '$lib/components/Announcement.svelte';
 
 	let { children } = $props();
 </script>
 
+<Announcement />
 {@render children()}