openai/gpt-chat-latest → B (2:3)

Side B removes the now-redundant `ExpandNewThreadForm` UI action end-to-end, simplifies the home page to server-render the collapsed compose/login state directly, and deletes the associated parsing and tests, reducing maintenance for a unified single-`+` flow. Side A fixes `thread_post_index` semantics by enforcing the invariant with `expect`, switching rank-history links to consistent 0-based URLs, updating documentation, and adding tests, but its scope is narrower than the architectural simplification in Side B.