Estimate preview no longer triggers document scroll

The preview panel now fills your viewport without pushing the outer page past its bounds.

Bug Fixes

The estimate preview's iframe, editor, and spreadsheet panels now occupy the full viewport as a fixed overlay. Previously, the layout calculation (h-[calc(100dvh-3.5rem)]) didn't account for the page's card chrome (header, borders, padding), so the document would grow beyond the viewport and the outer page would scroll. Now the preview container escapes the card entirely with fixed inset-0, and only the internal panels scroll — the document body stays fixed.