Phase 6: accordion in /learn; README feature notes; tests green

- collapse chapters/sections via styled <details> with chevrons
- identity testcontainers smoke passes on JDK 21
- README: student UX, comments, JDK note (logback ThrowableProxy in
  fat-jars → restart recovery), data dirs

web/src/main/resources/templates/learn/index.html

@@ -18,8 +18,12 @@
         Материалы ещё не добавлены.
     </div>
 
-    <div th:each="chapter : ${tree?.chapters}" class="card mb-4">
-        <div class="card-body">
+    <details th:each="chapter : ${tree?.chapters}" class="card mb-4 accordion" open>
+        <summary class="card-body acc-head">
+            <h2 class="section-title acc-title" th:text="${chapter.title}">Глава</h2>
+            <span class="acc-arrow" aria-hidden="true"></span>
+        </summary>
+        <div class="card-body acc-body">
             <div class="flex flex-wrap items-center justify-between gap-4 mb-4">
                 <h2 class="section-title" th:text="${chapter.title}">Глава</h2>
                 <div th:if="${currentUser != null and currentUser.admin}" class="flex flex-wrap gap-2">
@@ -35,20 +39,24 @@
 
             <p th:if="${#lists.isEmpty(chapter.sections)}" class="text-sm text-muted">Разделов пока нет</p>
 
-            <div th:each="section : ${chapter.sections}" class="mt-4">
-                <div class="flex flex-wrap items-center justify-between gap-4">
-                    <h3 class="group-title">
+            <details th:each="section : ${chapter.sections}" class="mt-4 sub-acc" open>
+                <summary class="sub-head">
+                    <h3 class="group-title m-0">
+                        <span class="sub-arrow" aria-hidden="true"></span>
                         <a th:href="@{/learn/{c}/{s}(c=${chapter.slug}, s=${section.slug})}" th:text="${section.title}">Раздел</a>
                     </h3>
+                </summary>
+                <div class="flex flex-wrap items-center justify-between gap-4 sub-actions">
+                    <div class="spacer"></div>
                     <div th:if="${currentUser != null and currentUser.admin}" class="flex flex-wrap gap-2">
                         <button class="btn btn-secondary btn-sm" type="button"
                                 th:data-modal="|modal-rename-section-${section.id}|" onclick="openModalData(this)">Переименовать</button>
                         <button class="btn btn-secondary btn-sm" type="button"
                                 th:data-modal="|modal-move-${section.id}|" onclick="openModalData(this)">Перенести статьи</button>
-                        <form th:action="@{/admin/content/sections/{id}/delete(id=${section.id})}" method="post"
+                        <form class="inline-form" th:action="@{/admin/content/sections/{id}/delete(id=${section.id})}" method="post"
                               onsubmit="return confirm('Удалить раздел со всеми статьями?')">
                             <input type="hidden" name="_csrf" th:value="${csrfToken}">
-                            <button class="btn btn-danger btn-sm" type="submit">Удалить</button>
+                            <button class="btn btn-danger btn-sm" type="submit">Удалить раздел</button>
                         </form>
                     </div>
                 </div>
@@ -105,8 +113,7 @@
                         </div>
                     </dialog>
                 </th:block>
-            </div>
-
+            </details>
             <th:block th:if="${currentUser != null and currentUser.admin}">
                 <dialog th:id="|modal-rename-chapter-${chapter.id}|" class="modal">
                     <div class="modal-header">
@@ -125,7 +132,7 @@
                 </dialog>
             </th:block>
         </div>
-    </div>
+    </details>
 
     <dialog id="modal-import" class="modal">
         <div class="modal-header">