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/+layout.svelte

@@ -0,0 +1,39 @@
+<script lang="ts">
+	import type { Snippet } from 'svelte';
+	import type { LayoutData } from './$types.js';
+
+	let { data, children }: { data: LayoutData; children: Snippet } = $props();
+
+	const navItems = [
+		{ href: '/admin/users', label: 'Users' },
+		{ href: '/admin/groups', label: 'Groups' },
+		{ href: '/admin/settings', label: 'Settings' }
+	] as const;
+</script>
+
+<div class="min-h-screen bg-background text-foreground">
+	<nav class="border-b border-border bg-card">
+		<div class="mx-auto flex max-w-6xl items-center gap-6 px-6 py-3">
+			<a href="/" class="text-sm text-muted-foreground hover:text-foreground">
+				&larr; Back to Dashboard
+			</a>
+			<span class="text-sm font-semibold text-card-foreground">Admin Panel</span>
+			<div class="flex gap-4">
+				{#each navItems as item}
+					<a
+						href={item.href}
+						class="rounded-md px-3 py-1.5 text-sm font-medium text-muted-foreground hover:bg-accent hover:text-foreground"
+					>
+						{item.label}
+					</a>
+				{/each}
+			</div>
+			<div class="ml-auto text-xs text-muted-foreground">
+				{data.user.displayName} (admin)
+			</div>
+		</div>
+	</nav>
+	<main class="mx-auto max-w-6xl p-6">
+		{@render children()}
+	</main>
+</div>