UI hooks for the browser loopback sign-in flow.
Implementations decide how to surface the authorize URL and whether to auto-open a browser. The Textual screen subclasses this; CLI / headless callers can supply a minimal stdout-based implementation.
The default base class implements the print-to-stdout fallback so it
works for headless tests and the -x non-interactive path; UI callers
override show_authorize_url.
