Login page quietly probes /auth/demo (debounced) as a valid email is
typed/pasted and reloads into the app on a match — no visible button.
Demo sessions default to the whole library, get a one-time shared-account
warning, never see the YouTube-connect onboarding/access UI or sync
actions, and admins get a demo whitelist + reset panel in Settings.
After the clean name/email sign-in, a wizard walks the user through granting
YouTube read (then optionally write) one step at a time, each with a plain
rationale and an up-front heads-up about Google's "unverified app" screen.
The visible step is derived from the granted scopes (can_read/can_write) so the
flow resumes correctly across the full-page consent redirect; it's dismissible
and reopenable from Settings -> Account, which now lists read and write as
separate, individually-grantable access rows.
