Learn UI: prev/next navigation, sidebar outline, editing on every page; merge import; defense date

- article page: ← / → navigation across the whole material tree (including
  between sections), sticky sidebar outline with the current article highlighted
- editing on chapter, section and article pages: rename, publish/hide, delete,
  create section/article, move articles while deleting a section; forms return
  to the page they were opened from (validated internal `back` parameter)
- article page opens the edit dialog directly via ?edit=1 (used by "Править"
  links on section/chapter pages)
- import: optional sync flag — with it stale sections/articles of archived
  chapters are deleted (previous behaviour), without it import only adds and
  updates, so material edited in the UI is not overwritten
- student area: lab page and lab cards now show the defense date next to the
  submission deadline
- fix SpEL crash on the student lab page (`and` with nullable strings)

web/src/main/resources/templates/student/lab.html

@@ -8,7 +8,11 @@
         <h1 class="page-title" th:text="|Лабораторная работа ${lab.number}. ${lab.title}|">ЛР</h1>
         <span th:if="${assignment != null}" class="badge badge-warn">Вариант <span class="font-mono" th:text="${assignment.variantNumber}"></span></span>
     </div>
-    <p class="text-sm text-muted mb-6" th:if="${deadlineMsk}">Срок сдачи: <span th:text="${deadlineMsk}"></span> МСК</p>
+    <p class="text-sm text-muted mb-6">
+        <span th:if="${deadlineMsk}">Срок сдачи: <span th:text="${deadlineMsk}"></span> МСК</span>
+        <span th:if="${deadlineMsk != null and defenseMsk != null}" class="nav-sep">·</span>
+        <span th:if="${defenseMsk}">Защита: <b th:text="${defenseMsk}"></b> МСК</span>
+    </p>
 
     <div th:if="${notice}" class="alert alert-success" th:text="${notice}"></div>
     <div th:if="${error}" class="alert alert-error" th:text="${error}"></div>

web/src/main/resources/templates/student/labs.html

@@ -16,7 +16,11 @@
                     <span th:unless="${c.submissionSeq}" class="badge badge-warn">не сдано</span>
                     <span th:if="${c.grade}" class="badge badge-ok">оценка: <span th:text="${c.grade}"></span></span>
                 </div>
-                <p class="text-sm text-muted mt-2" th:if="${c.deadlineMsk}" th:text="|Срок сдачи: ${c.deadlineMsk} МСК|">—</p>
+                <p class="text-sm text-muted mt-2">
+                    <span th:if="${c.deadlineMsk}" th:text="|Срок сдачи: ${c.deadlineMsk} МСК|">—</span>
+                    <span th:if="${c.deadlineMsk != null and c.defenseMsk != null}" class="nav-sep">·</span>
+                    <span th:if="${c.defenseMsk}" th:text="|Защита: ${c.defenseMsk} МСК|">—</span>
+                </p>
             </div>
         </div>
         <div th:if="${#lists.isEmpty(cards)}" class="card card-body text-muted">Опубликованных лабораторных пока нет.</div>