Content sort: autosave without reload; keep page state across reloads

- 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

web/src/main/resources/templates/fragments/base.html

@@ -11,6 +11,7 @@
     <script th:src="@{/js/htmx.min.js}" defer></script>
     <script th:src="@{/js/highlight.min.js}" defer></script>
     <script th:src="@{/js/tablekit.js}" defer></script>
+    <script th:src="@{/js/ui-state.js}" defer></script>
     <script>
         (function(){
             var t = localStorage.getItem('lms-theme') || 'light';