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/fragments/base.html

@@ -27,7 +27,7 @@
     <script th:src="@{/js/theme-init.js(v='36')}"></script>
     <script th:src="@{/js/app.js(v='36')}" defer></script>
     <script th:src="@{/js/ui-state.js(v='36')}" defer></script>
-    <script th:src="@{/js/form-feedback.js(v='37')}" defer></script>
+    <script th:src="@{/js/form-feedback.js(v='38')}" defer></script>
     <script th:src="@{/js/md-content.js(v='36')}" defer></script>
     <script th:src="@{/js/submit-guard.js(v='36')}" defer></script>
 </head>