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/fragments/base.html

@@ -4,11 +4,14 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title th:replace="${title}">LMS</title>
+    <link rel="icon" type="image/svg+xml" th:href="@{/favicon.svg}">
     <link rel="stylesheet" th:href="@{/css/app.css}">
     <script th:src="@{/js/htmx.min.js}" defer></script>
     <script>
         (function(){ var t = localStorage.getItem('lms-theme') || 'light'; document.documentElement.setAttribute('data-theme', t); })();
         function toggleTheme(){ var e=document.documentElement; var t=e.getAttribute('data-theme')==='dark'?'light':'dark'; e.setAttribute('data-theme',t); localStorage.setItem('lms-theme',t); }
+        function openModal(id){ var d=document.getElementById(id); if(d) d.showModal(); }
+        function closeModal(id){ var d=document.getElementById(id); if(d) d.close(); }
     </script>
 </head>
 <body>
@@ -16,13 +19,17 @@
     <header class="page-header">
         <div class="page-header-inner">
             <a th:href="@{/}" class="logo">LMS</a>
-            <nav class="nav" th:if="${currentUser != null}">
-                <a th:if="${currentUser.admin}" th:href="@{/admin/groups}" class="nav-link">Группы</a>
-                <a th:if="${currentUser.admin}" th:href="@{/admin/labs}" class="nav-link">Лабораторные</a>
-                <span class="nav-sep" th:text="${currentUser.name}">—</span>
+            <nav class="nav">
+                <a th:href="@{/learn}" class="nav-link">Материалы</a>
+                <th:block th:if="${currentUser != null}">
+                    <a th:if="${currentUser.admin}" th:href="@{/admin/groups}" class="nav-link">Группы</a>
+                    <a th:if="${currentUser.admin}" th:href="@{/admin/labs}" class="nav-link">Лабораторные</a>
+                    <span class="nav-sep" th:text="${currentUser.name}">—</span>
+                </th:block>
                 <button onclick="toggleTheme()" class="btn btn-secondary btn-sm">☀/🌙</button>
-                <form th:action="@{/logout}" method="post">
+                <form th:if="${currentUser != null}" th:action="@{/logout}" method="post">
                     <button class="btn btn-secondary" type="submit">Выйти</button>
+                <input type="hidden" name="_csrf" th:value="${csrfToken}">
                 </form>
             </nav>
         </div>