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/+page.svelte

@@ -1,5 +1,7 @@
 <script lang="ts">
-	// Placeholder — replaced in Phase 5 with the board layout
+	import type { PageData } from './$types.js';
+
+	let { data }: { data: PageData } = $props();
 </script>
 
 <svelte:head>
@@ -7,5 +9,20 @@
 </svelte:head>
 
 <main class="flex min-h-screen items-center justify-center bg-background text-foreground">
-	<h1 class="text-4xl font-bold">Web App Launcher</h1>
+	<div class="text-center">
+		<h1 class="text-4xl font-bold">Web App Launcher</h1>
+		{#if data.user}
+			<p class="mt-4 text-muted-foreground">
+				Welcome, {data.user.displayName}. No default board is configured yet.
+			</p>
+			<form method="POST" action="/auth/logout" class="mt-6">
+				<button
+					type="submit"
+					class="rounded-md bg-secondary px-4 py-2 text-sm font-medium text-secondary-foreground hover:bg-secondary/80"
+				>
+					Sign Out
+				</button>
+			</form>
+		{/if}
+	</div>
 </main>