Auth templates and widget nav links now point to
federation_url instead of blog_url, co-locating the
account UI with the auth system in the federation app.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Introduces a widget system where domains register UI fragments into
named slots (container_nav, container_card, account_page, account_link).
Host apps iterate widgets generically without naming any domain.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>