Add collapsible SiteWrapper header

[krowvin]

Summary

• add an opt-in collapseHeader prop to SiteWrapper
• hide the government banner and title/subtitle row in compact mode while retaining navigation and custom controls
• document the new API and cover expanded and collapsed rendering with regression tests

Closes #270

Checklist

• I updated docs/examples if the component API or behavior changed
• I added screenshots for UI changes when useful
• I applied exactly one version label: patch-bump, minor-bump, or major-bump

Screenshots

collapseHeader not set || collapseHeader={false}

collapseHeader={true} || collapseHeader

Release impact

minor-bump is appropriate because this adds a backward-compatible, opt-in component capability and preserves the existing expanded header by default.

Validation

• npm test -- --run (3 files, 10 tests)
• npm run lint
• npx prettier --check on changed files
• npm run build (library and docs assets built; the existing route-extraction step reported the missing @babel/parser dependency)
• live browser verification at 407x766 for expanded and collapsed states, including the mobile navigation overlay

[github-actions]

Please set a versioning label of either major-bump, minor-bump, or patch-bump to the pull request.