Signup Start

Sign-up entry point.

Two response modes, picked by query string:

1. NO ?confirmed (or ?confirmed != "1") -> return _signup_explainer_page, a self-contained HTML page that previews Google's account chooser + consent screen and lets the user click "Sign in with Google" to start the actual OAuth dance. The CTA on that page links to this same endpoint with ?confirmed=1.

2. ?confirmed=1 -> the original behavior: build the Google OAuth URL, set the state cookie, log signup_started/oauth_clicked funnel events, and 307 redirect to accounts.google.com.

The ?missing param is only honored on the explainer-page branch and is a comma-separated list of scope URLs the user failed to grant on a previous attempt; the page renders a warning banner with their plain- English labels.