block method

@override Future<ChatResult<void>> block(String userId)

Blocks a user, hiding their messages and preventing contact.

Two-way effect: incoming messages from userId are dropped on the server side, and the blocker can no longer DM the blocked user either. Backend emits user_blocked so the blocker's other devices update their contacts/DM lists. The blocked user is NOT notified.

Implementation

@override
Future<ChatResult<void>> block(String userId) async =>
    const ChatSuccess(null);