docs: add search functionality implementation plan
Detailed 11-task plan for implementing unified search:
1. Generate search index JSON template
2. Create shared search module (lazy-loading, filtering, Alpine components)
3. Create desktop search modal partial
4. Add search icon to header
5. Wire search event listener
6. Integrate search into mobile hamburger menu
7. Include modal and script in base template
8. Add i18n keys (EN/IT)
9. Refactor 404 page to use shared search
10. Build and manual test
11. Create feature branch and merge
Each task includes exact file paths, code blocks, commands, and expected outputs.
