feat(mvp): phase 3 - authentication system

Implement local auth flow: login, registration, logout, JWT access/refresh
tokens in HTTP-only cookies, hooks.server.ts middleware, guest mode support,
Superforms + Zod validation, and reusable auth/authorize middleware.

src/routes/+layout.svelte

@@ -0,0 +1,11 @@
+<script lang="ts">
+	import '../app.css';
+	import type { Snippet } from 'svelte';
+	import type { LayoutData } from './$types.js';
+
+	let { data, children }: { data: LayoutData; children: Snippet } = $props();
+</script>
+
+<div class="dark min-h-screen">
+	{@render children()}
+</div>