feat(onboarding): link Google 2FA help from Gmail app-password step

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>

app/src/main/res/values/strings.xml

@@ -152,6 +152,7 @@
     <string name="app_password_what_is">An app password is a one-off password that lets an app sign in to your account without your main password or a two-factor code.</string>
     <string name="app_password_warning">Store this app password carefully — it grants full access to your email. LibreMail keeps it only on this device.</string>
     <string name="app_password_open_page">Create an app password for %1$s</string>
+    <string name="app_password_2fa_help">How to turn on 2-Step Verification</string>
     <string name="app_password_open_failed">Couldn\'t open your browser</string>
     <string name="app_password_email">Email address</string>
     <string name="app_password_field">App password</string>