feat(mvp): phase 6 - admin panel

Add admin layout with auth guard, user management (CRUD + group membership),
group management, system settings (auth mode, registration, theme, healthcheck),
permission editor component, and global search API endpoint.

src/routes/admin/settings/+page.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	import type { PageData } from './$types.js';
+	import SettingsForm from '$lib/components/admin/SettingsForm.svelte';
+
+	let { data }: { data: PageData } = $props();
+</script>
+
+<svelte:head>
+	<title>System Settings — Admin</title>
+</svelte:head>
+
+<div>
+	<div class="mb-6">
+		<h1 class="text-2xl font-bold text-card-foreground">System Settings</h1>
+		<p class="mt-1 text-sm text-muted-foreground">Configure global application settings.</p>
+	</div>
+
+	<SettingsForm form={data.form} />
+</div>