feat: run Playwright accessibility scan on pull requests

[coseeian]

Goal

This PR wires the existing Playwright + axe-core a11y suite into CI so regressions get caught on PRs.

Notes for reviewers

• The scan currently covers the EN homepage. We'll expand coverage to other pages once we've monitored the CI stability on PRs from other contributors for a short period.
• By default, axe-core runs automatic accessibility check rules tagged for WCAG 2.0 and WCAG 2.1 at Level A and Level AA, along with selected best practices.
• A failed scan generates a GitHub Action summary (sample) containing the scan results and downloadable artifacts. The default message format should be understandable, but I'm happy to refine the report output for other contributors down the line if we see a need.
• The check is currently blocking - any new axe violations will fail the build. Once we confirm the CI is stable, I’d suggest we consider making this newly added action a required check in the branch protection rules, so PRs can’t be merged with violations.