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/student/labs.html

@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<html lang="ru" xmlns:th="http://www.thymeleaf.org" th:replace="~{fragments/base :: layout(~{::title}, ~{::#content})}">
+<head><title>Лабораторные — LMS</title></head>
+<body>
+<div id="content">
+    <h1 class="page-title">Лабораторные работы</h1>
+    <div class="grid grid-2">
+        <div th:each="c : ${cards}" class="card">
+            <div class="card-body">
+                <h2 class="section-title">
+                    <a th:href="@{/student/labs/{id}(id=${c.id})}" th:text="|Лабораторная работа ${c.number}. ${c.title}|">ЛР</a>
+                </h2>
+                <div class="flex flex-wrap gap-2 text-sm mt-2">
+                    <span th:if="${c.variantNumber}" class="badge badge-warn">Вариант <span class="font-mono" th:text="${c.variantNumber}"></span></span>
+                    <span th:if="${c.submissionSeq}" class="badge badge-ok">сдано (попытка <span th:text="${c.submissionSeq}"></span>)</span>
+                    <span th:unless="${c.submissionSeq}" class="badge badge-warn">не сдано</span>
+                    <span th:if="${c.grade}" class="badge badge-ok">оценка <span th:text="${c.grade}"></span></span>
+                </div>
+                <p class="text-sm text-muted mt-2" th:if="${c.deadlineMsk}" th:text="|Срок сдачи: ${c.deadlineMsk} МСК|">—</p>
+            </div>
+        </div>
+        <div th:if="${#lists.isEmpty(cards)}" class="card card-body text-muted">Опубликованных лабораторных пока нет.</div>
+    </div>
+</div>
+</body>
+</html>