fix(ui): correct admin mode detection logic

ui/src/routes/guests/+page.svelte

@@ -15,7 +15,9 @@
 		type AdminRsvp
 	} from '$lib/api';
 
-	const isAdmin = $derived(page.url.searchParams.get('admin') === '1');
+	// Guarded by `browser`: the site is prerendered and page.url.searchParams
+	// is inaccessible during prerender/SSR. Admin mode is a client-only concern.
+	const isAdmin = $derived(browser && page.url.searchParams.has('admin'));
 
 	let status = $state<'loading' | 'done' | 'error'>('loading');
 	let errorMessage = $state('');