Phase 6: Activity log and notifications

Add activity logging to all write API routes, notification system with
real-time delivery via Socket.IO, @mention parsing in comments, activity
feed sidebar on board page, per-card activity in card modal, and
notification bell with unread badge in the navbar.

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

src/routes/+layout.svelte

@@ -3,6 +3,7 @@
 	import type { Snippet } from 'svelte';
 	import { onMount } from 'svelte';
 	import { connectSocket, disconnectSocket } from '$lib/stores/socket.js';
+	import NotificationBell from '$lib/components/NotificationBell.svelte';
 
 	let { children, data }: { children: Snippet; data: { user: any; tenant: any } } = $props();
 
@@ -39,6 +40,7 @@
 							</a>
 						{/if}
 						<span class="text-white/80 text-sm">{data.user.name}</span>
+						<NotificationBell />
 						<button
 							onclick={async () => {
 								await fetch('/api/auth/logout', { method: 'POST' });