Lab assignment management UI and student variant view

- progress-service: POST /api/v1/assign (manual variant), GET /api/v1/assignments?labId=
  (enriched assignments), fixed enrich() to return real variant number/title/statement
- gateway: routes for /api/v1/assign and /api/v1/levels (exact paths)
- web: lab detail page shows students grouped with level and variant selects,
  group filter, distribute/redistribute buttons, source badge (auto/manual)
- web: student dashboard renders variant statement via libs/markdown (spoilers,
  code highlight, sanitized)
- academic-service: unquote YAML front-matter values (variant numbers were quoted)
- academic-service: LabLevelDetail.id for level references

web/src/main/resources/templates/fragments/base.html

@@ -17,7 +17,6 @@
         <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}" class="nav-link">Панель</a>
                 <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>