🔧 Cache Playwright browsers in CI

[jrosseel]

Summary

• restore the Playwright browser cache before affected Storybook regressions install Chromium
• key the cache by operating system and lockfile, with a safe OS-prefix fallback
• retain the unconditional install so an older restored cache is reconciled to the locked browser revision

Validation

• YAML parser
• git diff --check
• independent review: no findings
• documentation gate: PASS

Review

• independent PR based directly on develop; it does not include any open predecessor work
• resolves the repeated 19-minute Chromium download seen in run 32247063619 for subsequent eligible runs