[Website] Add custom error message when artifact is expired

[mho22]

Motivation for the change, related issues

A WordCamp Nice 2026 contributor asked why this was crashing :

https://playground.wordpress.net/wordpress.html?pr=9026

I found out the CI build artifact for that pull request was expired. To load that pull request, the author or a maintainer should push a new commit, rebase or rerun the CI Job to trigger a fresh CI build.

Implementation details

• Added a new custom error message.

Testing Instructions (or ideally a Blueprint)

1. npm run dev
2. Go to http://localhost:5400/website-server/wordpress.html?pr=9026

On Playground

Locally

[Copilot]

Pull request overview

Adds a clearer, actionable error message when a PR preview fails because the CI artifact has expired, so users understand how to regenerate the artifact and retry.

Changes:

• Handle artifact_expired errors with a custom message in the React PR preview form.
• Handle artifact_expired errors with a custom message in the static wordpress.html PR previewer page.
• Updates the HTML doctype casing/formatting.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/playground/website/src/github/preview-pr/form.tsx	Adds a dedicated artifact_expired branch with a tailored error message in the web UI.
packages/playground/website/public/wordpress.html	Adds the same artifact_expired message for the static previewer and changes the doctype line.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The PR description focuses on adding a custom error message, but this diff also changes the doctype line (casing/formatting). If this isn’t required for the error-message behavior, consider reverting it to keep the PR minimal and avoid unrelated churn/merge conflicts.

[mho22]

This seems to be produced by the npm run format pre-commit hook.

[JanJakes]

@mho22 Looks and works good, thanks! I noticed an unrelated typo in the existing code, so I pushed one more commit to address that.

[mho22]

@JanJakes Thanks!