devops: run axe-core a11y suite in CI on every PR

[nanaf6203-bit]

Summary

Axe e2e runs locally but not in CI.

Affected area

• Add CI step that runs tests/e2e/accessibility.spec.ts on every PR and fails build on regression.

Acceptance criteria

• Implement the change described above.
• Add or update unit/e2e tests locking in the new behaviour.
• Update README / docs to reflect the new state.

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Filed automatically as part of the PropChain code-review sweep.