Phase 5: Real-time updates via Socket.IO

Wire Socket.IO broadcast from all write API routes so board changes
propagate instantly to other connected clients. Add session-based auth
on socket connections, in-memory presence tracking, and a presence UI
in the board header showing who else is viewing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/+layout.svelte

@@ -1,8 +1,17 @@
 <script lang="ts">
 	import '../app.css';
 	import type { Snippet } from 'svelte';
+	import { onMount } from 'svelte';
+	import { connectSocket, disconnectSocket } from '$lib/stores/socket.js';
 
 	let { children, data }: { children: Snippet; data: { user: any; tenant: any } } = $props();
+
+	onMount(() => {
+		if (data.user) {
+			connectSocket();
+			return () => disconnectSocket();
+		}
+	});
 </script>
 
 <div class="min-h-screen bg-gray-50">