openai/gpt-chat-latest → B (2:3)

Side B introduces a centralized `util/get-base-url` used across multiple OAuth/callback integrations (Linear, Spotify, Twitter, YouTube, login, OAuth) and updates the frontend to derive URLs from `window.location.origin`, eliminating scattered hardcoded hostnames and improving deployment portability. Side A contains important test infrastructure fixes—correcting mock OAuth request handling (`getRequestBody`, query parsing, null checks, redirects, exception handling) and stabilizing Playwright auth tests—but its impact is primarily confined to the test suite rather than the application's runtime design.