Replace per-row checkbox + numeric Order inputs with a drag-to-reorder
list of selected overlays plus a native <select> for adding more.
Native HTML5 DnD; no library, no JS-disabled fallback. Server contract
unchanged (overlay_ids in DOM order; existing fallback_position branch
absorbs the omitted overlay_position_<id> fields).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
