auth.ts demo

[hbenl]

This PR demonstrates auth.ts successfully fetching the auth tokens after a login in an external browser.
STR:

• start the browser in a shell, it will print a message "openExternalBrowser: https://app.replay.io/api/browser/auth?key=..." in the shell
• open that URL in an external browser, click "Sign in with Google" and login
• after a few seconds you'll see messages "setReplayRefreshToken: ..." and "setReplayUserToken: ..." in the shell

Don't merge this PR: a browser built with it will poll our backend for 2 minutes to get the result of the external login flow every time it is started.