Gmail's app-passwords page rejects accounts that don't have 2-Step
Verification enabled, and the setup screen's intro text names that
prerequisite without giving the user any way to act on it. Add a
nullable MailProvider.twoFactorHelpUrl (set only for Gmail, to
Google's "Turn on 2-Step Verification" article) and surface it as a
second outlined button under the existing app-password link, reusing
the same UriHandler + snackbar failure plumbing. Yahoo and iCloud
screens are unchanged.
Closes#98
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>