fix: validate grade input and save modal without reload

- require a score and return a clear message for empty/non-numeric values
- handle unreadable API request bodies instead of falling back to a bare 400
- submit the grade modal via fetch, show errors in place and update the cell
- keep the page scroll position; restore the form feedback tests

web/src/main/resources/templates/admin/group.html

@@ -159,7 +159,8 @@
             <input type="hidden" name="commentsJson" id="grade-comments" value="[]">
             <div><label class="label">Студент</label><input class="input" id="grade-who" readonly></div>
             <div><label class="label" for="grade-score">Балл (0–5)</label>
-                <input class="input" id="grade-score" name="score" type="number" min="0" max="5" step="0.1"></div>
+                <input class="input" id="grade-score" name="score" type="number" min="0" max="5" step="0.1" required
+                       data-error-required="Укажите оценку"></div>
             <div><label class="label" for="grade-general">Общий комментарий по лабораторной</label>
                 <textarea class="input" id="grade-general" name="generalComment" rows="4"
                           placeholder="Итоговый комментарий по работе в целом…"></textarea></div>
@@ -345,7 +346,7 @@
     <div class="dlg-body" id="assign-modal-body"></div>
 </dialog>
 
-<script th:src="@{/js/admin-group.js(v='37')}" defer></script>
+<script th:src="@{/js/admin-group.js(v='38')}" defer></script>
 </div>
 </body>
 </html>