Add WordPress-style body classes to Faust pages #2682

WordPress themes get classes like home, single-post, postid-123 and logged-in on <body> from body_class(). Faust pages don't, so CSS ported from a theme loses those hooks. The Toolbar already adds admin-bar by hand and notes this could become a Faust hook (Toolbar.tsx).

• Work out the classes from the seed node Faust already loads, with an opt-in hook such as useBodyClass().
• Have the Toolbar use it for admin-bar.
• Matching core's body_class() exactly (including its filters) is out of scope, because that needs WordPress's main query.

Open question: which classes are worth supporting first?