Preview/PR builds don't have access to GitHub Secrets, so the OAuth
client IDs are empty. Previously this caused a runtime crash from
keys.dart throwing on empty values. Now we detect missing secrets
and force simulate mode, which already fully supports fake auth
and uploads.
Also fixes a latent bug where forceLogin() would crash with
LateInitializationError in simulate mode since _helper is never
initialized when OAuth setup is skipped.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
