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/learn/section.html

@@ -0,0 +1,23 @@
+<!DOCTYPE html>
+<html lang="ru" xmlns:th="http://www.thymeleaf.org" th:replace="~{fragments/base :: layout(~{::title}, ~{::#content})}">
+<head><title th:text="${section.title} + ' — LMS'">Раздел</title></head>
+<body>
+<div id="content">
+    <a th:href="@{/learn}" class="nav-link">← Все материалы</a>
+    <h1 class="page-title mt-4" th:text="${section.title}">Раздел</h1>
+    <p class="text-sm text-muted mb-6">Глава: <span th:text="${chapter.title}">—</span></p>
+    <div class="card card-flush">
+        <table>
+            <thead><tr><th>Статья</th></tr></thead>
+            <tbody>
+            <tr th:each="article : ${section.articles}">
+                <td><a th:href="@{/learn/{c}/{s}/{a}(c=${chapter.slug}, s=${section.slug}, a=${article.slug})}"
+                       th:text="${article.title}">—</a></td>
+            </tr>
+            <tr th:if="${#lists.isEmpty(section.articles)}"><td class="empty">Статей пока нет.</td></tr>
+            </tbody>
+        </table>
+    </div>
+</div>
+</body>
+</html>