qa(ui): add Playwright + axe UI/QA coverage

[UberMetroid]

Summary

• add Playwright 1.50 and axe 4.10 UI/QA coverage against external STUDIO_URL
• cover root, PWA manifest, service worker, PIN auth, leaderboard round trip, accessibility, and game-board visuals
• add Alpine container-backed GitHub Actions workflow

Verification

• npm install
• npx playwright install chromium
• Alpine Containerfile build
• npm test (8 passed)

Limitations

• the app has no leaderboard UI, so leaderboard display and axe coverage use the authenticated JSON endpoint rendered in Chromium
• axe findings are attached but only fail when A11Y_STRICT=true
• service-worker coverage validates delivery and cache headers, not offline cache installation
• the round-trip test writes one synthetic entry under a unique category
• the requested Playwright 1.50.x dependency reports two high-severity npm audit findings