Function authorize 

pub async fn authorize(
    config: &OAuthConfig,
    cancel: Receiver<bool>,
    clock: &dyn Clock,
) -> Result<OAuthTokens, OAuthError>

Open the user’s browser, wait for the OAuth callback, and exchange the authorization code for tokens.

Flow:

1. Generate PKCE verifier + challenge
2. Open browser to auth_url with the required parameters
3. Spawn a one-shot HTTP server on localhost:{redirect_port}
4. Wait for the callback with the authorization code
5. Exchange the code for tokens at token_url