The first desktop release with accounts opens straight into the library
with nothing to say what changed. A three-step card now says the local
library is untouched, offers sign-in, and points at Replays. It shows once
per install, waits for any update dialog and for launches that open a file
or link, skips the library step on an empty install, and What's new leads
back to it.

The seen flag is a new AppPreferences field whose default (not seen)
reproduces the old behavior, so stores written before it read correctly
without a migration; a test reads such a record.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>