Add automated a11y CI gate using jest-axe (WCAG 2.1 AA) #13005

Problem / Opportunity

Open Library has no automated accessibility (a11y) testing in CI. Accessibility regressions can land silently — a Lit component can ship with broken ARIA without any check failing. We have 17 open `Theme: Accessibility` issues and no tooling that would have caught them.

Opportunity: Wire jest-axe into the existing JavaScript test suite so that any PR introducing a WCAG 2.1 AA violation in a Lit component is blocked at CI.

Proposal

Add jest-axe as a devDependency and write WCAG 2.1 AA tests for the three Lit components that currently have manual ARIA attributes but no automated coverage:

• `openlibrary/components/OlPopover.js`
• `openlibrary/components/OlOptionsPopover.js`
• `openlibrary/components/OlToast.js`

Wire `npm run test:a11y` into `.github/workflows/javascript_tests.yml` so violations block merge.

This is Phase 1 of a broader a11y tooling plan. Phase 2 (Playwright + axe-core full-page scans) and Phase 3 (Lighthouse CI) are follow-on work.

Breakdown

• Install `jest-axe` as devDependency
• Write tests for `OlPopover`, `OlOptionsPopover`, `OlToast` (default + open states)
• Add `test:a11y` script to `package.json`
• Wire into `javascript_tests.yml` CI
• Verify all 477 existing tests still pass
• Verify 0 WCAG violations on current components

Success Criteria

• `npm run test:a11y` passes with 0 violations on merge
• CI blocks any future PR that introduces a new WCAG violation in a tested component
• All existing tests continue to pass