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>

server.js

@@ -75,6 +75,11 @@ io.on('connection', (socket) => {
 	const user = socket.data.user;
 	console.log(`Socket connected: ${socket.id} (user: ${user?.name ?? 'anon'})`);
 
+	// Join user-specific room for notifications
+	if (user) {
+		socket.join('user:' + user.id);
+	}
+
 	// Track which boards this socket has joined (for cleanup on disconnect)
 	const joinedBoards = new Set();