v2.7.2: lazy-mounted editor previews

Fixed

Scrolling was slow, or stalled entirely, while blocks showed their visual preview.

Every Page Block in preview mode is a separate document carrying the whole theme stylesheet set — measured at roughly 25 stylesheets and 2,200 CSS rules to style about 30 elements, duplicated per block. A page of eight blocks therefore kept around 200 stylesheets and 17,600 rules alive at once, each in its own compositing context, and scrolling paid for all of it. Switching a block to code mode destroyed its frame, which is why code mode always felt fine.

Previews are now mounted only while within 800px of the viewport and unmounted beyond it. On a 12-block page that means at most 3 frames live instead of 12, and 75 stylesheets instead of 300. A holder element retains each frame's measured height whether or not it is mounted, so unmounting never collapses a block or moves the scroll position.

Preview fidelity is unchanged: a mounted frame still loads the full theme stylesheet set and theme.json global styles.