Fix non-serializable accessFilter in admin load, add error boundary and tests

src/routes/(protected)/admin/+error.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+	import { page } from '$app/stores';
+	import Window from '$lib/components/containers/Window.svelte';
+</script>
+
+<Window>
+	<div class="flex flex-col items-center gap-4 p-6 text-center">
+		<h1 class="text-xl font-bold text-gray-900 dark:text-gray-100">
+			Admin Error ({$page.status})
+		</h1>
+		<p class="max-w-md text-sm text-gray-600 dark:text-gray-400">
+			{#if $page.error?.message}
+				{$page.error.message}
+			{:else}
+				An unexpected error occurred while loading this admin page.
+			{/if}
+		</p>
+		<a
+			href="/admin"
+			class="mt-2 border border-gray-200 px-4 py-2 text-sm font-medium text-gray-900 hover:bg-gray-50 dark:border-gray-700 dark:text-gray-100 dark:hover:bg-gray-800"
+		>
+			Back to Admin
+		</a>
+	</div>
+</Window>