Fix live code examples in Firefox-based browsers

[petyosi]

Summary

Fixes #1408.

The docs live-code preview iframe now mounts through a browser-agnostic iframe document path instead of depending on a Firefox-specific load/ref branch. The preview iframe also gets an explicit empty document and a full-height document root so examples that use height: '100%' can measure correctly in Firefox-based browsers like Zen.

Validation

• User verified the live examples work in Zen.
• pnpm exec oxfmt --check apps/virtuoso.dev/src/components/LiveCodeBlock/LiveCodeBlock.tsx apps/virtuoso.dev/src/components/LiveCodeBlock/iframe-style.css

Note: the pre-commit lint hook is currently blocked by the existing oxlint config error Rule 'prop-types' not found in plugin 'react', so the commit was created with hooks skipped after the scoped formatter check passed.

[changeset-bot]

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Latest commit: a4b984a

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