Add production Administration and Storefront builds to the scaffolded PR pipeline

[lasomethingsomething]

User story

As a dev on a Shopware project, I want the CI pipeline that project create scaffolds to build the Administration and Storefront in production mode on every pull request, so that a change which breaks either build fails before it is merged, instead of after.

Why: the 6.6 → 6.7 upgrade report lists this as P0: "Production administration and Storefront builds in pull-request CI." In that migration, administration ES builds were disabled after the merge, re-enabled with a component fix, then disabled again: 4of 11 post-merge stabilization commits. The report's conclusion is that a production administration build was not a reliable PR gate.

The scaffolded pipeline today runs project validate and a code-style check, but never builds assets, so it covers the PHP steps of the report's ten-step upgrade gate and skips the two build steps.

Acceptance criteria

Up for discussion:

• A scaffolded GitHub or GitLab pipeline fails the PR when the production Administration or Storefront build fails.
• The build runs as its own job, so validation results still come back fast.
• Documented in the project create docs, including how to remove it.

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.