Add a toggleable live preview panel to both editors

Hidden by default, shares horizontal space with the body editor once
opened rather than a separate section, and only requests the iframe's
src on first toggle -- no preview traffic at all until someone
actually wants to see it. Refreshes automatically on every successful
autosave; a separate force-render button re-attempts a save-and-
refresh cycle immediately, for when the normal 7-second cycle has
been interrupted (a lock-lost error, a transient failure) and waiting
for the next tick isn't good enough.

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