Keep pinch zoom optical instead of reflowing the workspace

The workspace shell followed window.visualViewport so it could stay above the
on-screen keyboard. A pinch zoom also shrinks and moves the visual viewport, so
the app re-laid itself out inside the zoomed region and looked like page zoom.
Follow the visual viewport only at unit scale; while pinch zoomed, keep the
layout viewport and let the browser magnify it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>