Thread Subscription for Proof Comment Threads (Auto-follow + Manual Follow/Unfollow)

In Workfront Proof, add the ability for a user to subscribe (follow) a specific comment thread so they receive notifications for all subsequent replies in that thread, even if they did not author the original comment and are not explicitly @mentioned.

• When a user is replied to within a thread (or is otherwise “looped in”), Workfront can optionally auto-subscribe them to that thread (configurable per user notification preferences).
• Provide a clear Follow/Unfollow thread control directly on the thread UI.
• Notifications should respect existing Proof notification channels (email/in-app) but add a new event type: “New reply in followed thread.”