openai/gpt-chat-latest → B (2:1)

Side B fixes a concrete concurrency issue by precomputing `room_members_section_markup`, dropping the `tokio::sync::RwLock` read guard before expensive page layout/stringification, and avoiding reader stalls when a writer is waiting. Side A is primarily a broad refactor that unifies public and room new-thread UI IDs, actions, and markup; while it reduces duplication and standardizes behavior, it is mostly structural and also changes UI behavior rather than addressing a clear correctness or performance problem.