The visual page builder, in its own live_session.

:section is page | layout | error-page | component; the canvas is a second LiveView the chrome embeds in an iframe, addressed by its session token.

These are :admin_standalone rather than :admin because the builder is the one admin page whose <head> is not the admin's usual head — it carries the in-browser Tailwind JIT and the shim that nonces what that JIT and CodeMirror create. The root layout builds <head> on a dead render only, so a live navigation into the builder would hand it the previous page's head. The separate session is what makes arriving here a fresh document; navigation BETWEEN these routes stays live and fast.