openai/gpt-chat-latest → B (2:1)

Side B makes a lasting functional improvement: `post_choose_username` now returns HTML fragments for success/error instead of redirects, and the shared Poem JS reads non-empty responses to morph a form's `innerHTML`, enabling inline auth feedback while preserving existing empty-response behavior for other forms. Side A primarily removes the duplicate `server/src/html/forum.rs` and establishes `forum/mod.rs` with split modules to eliminate module-root ambiguity, which is valuable maintenance but mostly a structural cleanup rather than new user-facing behavior.