fix(Editor): #3587909 Force cross-frame layout reflow in vh-unit detection loop

Fix infinite height scaling

AI-Generated: Yes (Used Claude Code to write issue and code).

Testing instructions

1. Create a code component with a min-height: 100vh element (e.g., a <div style="min-height: 100vh; background: red;">test</div>)
2. Add it to a Canvas page and open the editor
3. Confirm the preview iframe renders at a stable height instead of growing unbounded
4. In DevTools, inspect the div inside the preview iframe — confirm data-canvas-preview-max-height attribute is present with a sane pixel value

Closes #3587909