openai/gpt-chat-latest → A (9:1)

Side A consolidates duplicated test infrastructure into shared helpers (`test.common` and `test.oauth`), introducing reusable functions like `run-cargo-build-release!`, `slug-server-env`, `complete-registration!`, and configurable mock OAuth behavior while updating multiple test suites to use them. This reduces duplication, centralizes maintenance, and also standardizes HTTP timeouts and multi-user OAuth testing; Side B is primarily a UI cleanup that removes the form toggle and tweaks placeholders/button text without adding comparable lasting functionality.