{{-- Full-width search bar that covers the header. A [popover] (top layer, so it sits over the sticky header with no z-index juggling; Escape and click-away close come for free). The header's search button opens it via popovertarget. The `nav-search` controller measures the header on open (→ --search-overlay-h, since the header isn't a fixed height below `lg`) and focuses the field. Fade transition lives in app.css (#search-overlay). --}}