- drag & drop now saves in the background (700 ms debounce) without reloading:
sections stay open, scroll position is kept; on failure it retries every 3 s,
and navigator.sendBeacon flushes unsaved changes on page leave
- removed the explicit save button (autosave only), status indicator shows
«Изменения не сохранены… / Сохранение… / Порядок сохранён»
- new global ui-state.js: restores open <details> and scroll position after any
reload/redirect within the same page (applies to all pages, not just /learn)
- auto-expand a collapsed chapter/section while hovering during drag
