feat: create shared search module with lazy-loading and Alpine components
- Implement loadSearchIndex() for async JSON fetching and caching
- Implement filterArticles(query, articles) with case-insensitive search (max 5 results)
- Register three Alpine.js components: searchOverlay, mobileSearch, notFoundPage
- Support desktop modal, mobile menu, and 404 page search integration
- Include Escape key handling and index lazy-loading optimizations
