feat:move auth demo to main

src/routes/+page.svelte

@@ -1,2 +1,14 @@
-<h1>Welcome to SvelteKit</h1>
-<p>Visit <a href="https://svelte.dev/docs/kit">svelte.dev/docs/kit</a> to read the documentation</p>
+<script lang="ts">
+	import { enhance } from '$app/forms';
+	import type { PageServerData } from './$types';
+
+	let { data }: { data: PageServerData } = $props();
+</script>
+
+<h1>Welcome to Serengo</h1>
+<p>Hi, {data.user.username}!</p>
+<p>Your user ID is {data.user.id}.</p>
+
+<form method="post" action="/logout" use:enhance>
+	<button>Sign out</button>
+</form>