move better-auth demo to auth dir

src/routes/auth/+page.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+	import { enhance } from '$app/forms';
+	import type { PageServerData } from './$types';
+
+	let { data }: { data: PageServerData } = $props();
+</script>
+
+<h1>Hi, {data.user.name}!</h1>
+<p>Your user ID is {data.user.id}.</p>
+<form method="post" action="?/signOut" use:enhance>
+	<button class="rounded-md bg-blue-600 px-4 py-2 text-white transition hover:bg-blue-700"
+		>Sign out</button
+	>
+</form>