test: Add accessibility coverage with webship-js

Problem/Motivation

Nothing checks the Website API Starter's pages for accessibility, so regressions in landmarks, heading order, labels, link names or contrast are only found by hand.

Steps to reproduce

1. Install the site template and visit any of its pages.
2. There is no automated check for accessibility issues (landmarks, heading order, labels, link names, contrast).

Proposed resolution

Add a webship-js accessibility feature that runs the axe-core backed steps over the template's pages, and run the suite on webship-js 2.0.6.

Remaining tasks

• ✅ Add the webship-js accessibility feature
• ❌ Run the suite on webship-js 2.0.6
• ❌ Reviewed by a human
• ❌ Code review by maintainers

User interface changes

N/A

API changes

N/A

Data model changes

N/A

AI-Generated: Yes