[prithq]

Issue:

Fixes #4321

The preview iframe was loading the generated sketch HTML through a blob: URL.
Because the document was loaded from a blob: URL, requests made by the sketch
were missing the expected Referer header.

This PR changes the preview to use iframe.srcdoc for the generated HTML instead
of creating a Blob URL for the entire HTML document.

Demo:

Changes:

Load the generated sketch HTML using iframe.srcdoc.

• Remove the Blob URL lifecycle used for the generated index.html.
• Keep Blob URLs for individual local resources such as JavaScript files.
• Preserve the existing Safari-compatible deferred rendering behavior.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines