feat: submit all modal forms without page reload

- add a generic AJAX submitter for forms inside dialogs
- keep modal values and show field errors on server validation failure
- soft-refresh the page content on success, preserving scroll, search and filters
- share toast and re-init hooks for tables, markdown, sections and group page
- require passwords and add minlength/match validation on auth forms

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

@@ -278,7 +278,8 @@
             <p class="hint">Если студент уже в другой группе — он будет переведён в эту.</p>
             <button class="btn btn-secondary" type="submit" th:disabled="${#lists.isEmpty(candidates)}">Зачислить существующего</button>
         </form>
-        <form class="form-stack mt-4" th:action="@{/admin/groups/{gid}/students/create(gid=${group.id})}" method="post">
+        <form class="form-stack mt-4" th:action="@{/admin/groups/{gid}/students/create(gid=${group.id})}" method="post"
+              data-success="Студент создан. Пароль показан ниже.">
             <input type="hidden" name="_csrf" th:value="${csrfToken}">
             <div class="row g-3">
             <div class="col-md-6"><label class="label" for="stNo">Студенческий номер (логин)</label>
@@ -346,7 +347,7 @@
     <div class="dlg-body" id="assign-modal-body"></div>
 </dialog>
 
-<script th:src="@{/js/admin-group.js(v='38')}" defer></script>
+<script th:src="@{/js/admin-group.js(v='39')}" defer></script>
 </div>
 </body>
 </html>