Phase 2+5: lab submissions and student lab flow

- progress: submissions table+V4; SubmissionService (deadline-gated submit,
  unlimited resubmits, per-student history, zip download); files on disk
  data/submissions/{student}/{lab}/{seq}/
- student API: POST submit (multipart), GET me submissions, download; admin
  GET lab submissions + admin download (zip pack)
- progress enrich(): labId in MeAssignment; web DTO aligned
- web: /student dashboard (avg/best/worst cards), /student/labs cards,
  /student/labs/{id}: condition md, variant block, submit form, history;
  'Лабораторные' link in nav for students
- admin: submissions column on lab assignment table + download link
- fixed long-standing template bug: broken @{} expression (missing }) in
  admin/lab and student/lab

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

@@ -26,6 +26,7 @@
                 <th:block th:if="${currentUser != null}">
                     <a th:if="${currentUser.admin}" th:href="@{/admin/groups}" class="nav-link">Группы</a>
                     <a th:if="${currentUser.admin}" th:href="@{/admin/labs}" class="nav-link">Лабораторные</a>
+                    <a th:unless="${currentUser.admin}" th:href="@{/student/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>