fix: install complete Playwright runtime dependencies

[jmagar]

Summary

• install Playwright’s complete Chromium runtime dependency set on Ubuntu 26.04 runners
• use the supported Ubuntu 24.04 dependency manifest without downloading/replacing the cached browser
• update the CI contract test to enforce dependency installation and preserve the cached-browser invariant

Root cause

The new TypeScript runner image only received libnspr4 and libnss3; Chromium then failed before browser tests with libatk-1.0.so.0: cannot open shared object file.

Verification

• PLAYWRIGHT_HOST_PLATFORM_OVERRIDE=ubuntu24.04-x64 pnpm exec playwright install-deps chromium --dry-run
• cargo test -p labby --test ci_changed_paths --all-features ci_workflow_uses_changed_path_classifier_and_stable_gate -- --exact
• cargo clippy -p labby --test ci_changed_paths --all-features -- -D warnings
• cargo fmt --all -- --check
• actionlint .github/workflows/*.yml

[jmagar]

Superseded by #305, which includes this Playwright runtime dependency fix together with the persisted Code Mode MCP UI implementation and follow-up reconciliation hardening.