feat: preserve form values and show field errors on submit

- keep submitted values and API field violations across redirects
- reopen the failed modal and highlight invalid fields
- show inline HTML5 validation messages instead of browser bubbles
- keep content-admin errors on the originating page

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

@@ -345,7 +345,7 @@
     <div class="dlg-body" id="assign-modal-body"></div>
 </dialog>
 
-<script th:src="@{/js/admin-group.js(v='36')}" defer></script>
+<script th:src="@{/js/admin-group.js(v='37')}" defer></script>
 </div>
 </body>
 </html>