feat: add client-side search and image lightbox

[tallnato]

Summary

• Adds client-side search across all 5 collections (lightshows, locksounds, boombox, wraps, hornsounds) using a build-time JSON index and simple term matching
• Integrates GLightbox (3.3.0, ~4KB gzipped) for wrap gallery images with keyboard navigation (arrow keys, Escape)
• Search input in site header with dropdown results showing item title and collection
• Wrap page images are auto-wrapped in lightbox links via DOM manipulation

Acceptance Criteria

• Search input in header finds items across all collections by title, description, and tags
• Search results link to the correct item pages
• Image lightbox opens full-size images on wrap item pages
• Lightbox supports keyboard navigation
• Both features work in light and dark mode (uses CSS custom properties)

Test plan

• Verify search.json is generated at build time with all collection items
• Type in the search box and confirm results appear for titles, descriptions, and tags
• Click a search result and verify it navigates to the correct item page
• Visit a wrap item page and click an image to confirm lightbox opens
• Test arrow keys and Escape in the lightbox
• Test in both light and dark modes

Closes NAT-84

🤖 Generated with Claude Code

[tallnato]

Closing — this work was already merged in PR #38 (de51ac4) and subsequent fixes in #45. The search and lightbox features are live on main.