feat: add docusaurus-lunr-search support

[philip]

Summary

• Adds docusaurus-lunr-search (3.6.0) for offline, client-side full-text search
• Registers the plugin in docusaurus.config.ts
• Styles the search input and results dropdown to match site colors

Notes

• Search index is built at npm run build time, so npm run dev shows "Loading..." in search box
• Project is abandoned but is functional. Used as per AppKit docs, did not explore alternatives

Test plan

• npm run build succeeds
• Search box visible and readable in both light and dark mode
• Search results return correct pages with highlighted terms
• npm run test passes