Rewrite /api/media-items/search endpoint to detect HTMX requests and return appropriate response format. The endpoint now checks for HX-Request header and routes to HTML renderer or JSON handler accordingly.
- Check HX-Request header to detect HTMX requests
- Return HTML via BooksGrid template for HTMX requests
- Return JSON for API clients (existing behavior)
- Add handleSearchHTML function for HTML rendering
- Use shared MediaHandler.ExecuteSearch method
- Eliminates previous issue where JSON was rendered in browser