UI rework: modal dialogs, inline lab editing, manual creation, content management

- Dashboard: real stats (groups, students, labs, average grade), labs/groups
  summary lists, no placeholder tile
- Lab page: view + edit combined — metadata form in modal, condition rendered
  as markdown, per-level variant tables with add/edit/delete dialogs,
  level rename/delete, student level/variant selectors with group filter
- Labs list: create-lab dialog with manual level entry (code; title lines)
- Groups: create-group dialog, add-student dialog (auto password, toast card)
- Content: public /learn reader is the single place; admin buttons (new
  article with chapter/section create-or-select, rename/delete chapters and
  sections, move articles between sections, import zip) appear for admins;
  removed separate 'Контент' admin section
- Buttons use btn classes instead of nav-link
- Favicon: generated SVG book icon
- Security: CSRF synchronizer token interceptor (replaced spring-security in
  web which broke logback under nested jars), session regenerated on login,
  hardened session cookie, gateway actuator limited to health,info
- Jackson 3: nullable-safe request DTOs for labs (FAIL_ON_NULL_FOR_PRIMITIVES)
- academic: POST /labs (manual create with levels), level add/rename/delete,
  content chapters/sections CRUD, article create/move/delete endpoints

web/src/main/resources/templates/admin/groups.html

@@ -11,6 +11,7 @@
             <h2 class="section-title">Создать группу</h2>
             <div><label class="label" for="name">Название</label><input class="input" id="name" name="name" placeholder="ПИ-11БО" required></div>
             <button class="btn btn-primary" type="submit">Создать</button>
+        <input type="hidden" name="_csrf" th:value="${csrfToken}">
         </form>
         <form class="card form-card form-stack" th:action="@{/admin/groups/import}" method="post" enctype="multipart/form-data">
             <h2 class="section-title">Импорт студентов</h2>
@@ -18,6 +19,7 @@
             <div><label class="label" for="file">Файл со списком</label><input class="input" id="file" name="file" type="file" accept=".xls,.xlsx,.csv" required></div>
             <div><label class="label" for="groupName">Название группы (необязательно)</label><input class="input" id="groupName" name="name" placeholder="Возьмём из файла"></div>
             <button class="btn btn-primary" type="submit">Импортировать</button>
+        <input type="hidden" name="_csrf" th:value="${csrfToken}">
         </form>
     </div>
     <div class="card card-flush mt-8">