fix(web): keep the grid preview pinned to the section it is previewing

[TheMaestr-o]

What does this PR do?

Keeps the admin's grid preview pinned to the section it is previewing, instead of drifting back to the hero.

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Base: feat/home-grid-polish (#136) — merge that first. No issue of its own.

The preview scrolled itself to the grid with a single setTimeout(120), which raced the page it was measuring. At 120ms the globe, the fonts and the card photos are still landing, so the offset it computed was stale a moment later and the frame settled back on the hero — intermittently, and more often on a slow machine, which is exactly what made it look like it worked.

It also never ran again. Switching the editor between desktop, tablet and mobile reflows the document to a different height, but the scroll position stayed where the old layout had put it, so the two panes could disagree about what they were showing.

Fix

The section is pinned on mount and re-pinned on load, on resize, and whenever a ResizeObserver sees the document or the section change size — all coalesced through one animation frame.

Verification

• 15 runs at normal, 4× and 8× CPU throttling: identical every time, both panes in sync, 97% of the section in frame.
• Desktop → tablet → mobile → desktop re-pins each time (scrollY 558 → 732 → 549 → 558) with section top = 0 in every mode. Before the fix scrollY stayed at 558 throughout.