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/change_password.html

@@ -14,8 +14,8 @@
         </p>
         <form class="card form-card form-stack" th:action="@{/change-password}" method="post">
             <div><label class="label" for="old">Текущий пароль</label><input class="input" id="old" name="old" type="password" autocomplete="current-password" required></div>
-            <div><label class="label" for="new">Новый пароль</label><input class="input" id="new" name="new" type="password" autocomplete="new-password" required><p class="hint">Минимум 8 символов.</p></div>
-            <div><label class="label" for="new2">Повторите новый пароль</label><input class="input" id="new2" name="new2" type="password" autocomplete="new-password" required></div>
+            <div><label class="label" for="new">Новый пароль</label><input class="input" id="new" name="new" type="password" autocomplete="new-password" minlength="8" required><p class="hint">Минимум 8 символов.</p></div>
+            <div><label class="label" for="new2">Повторите новый пароль</label><input class="input" id="new2" name="new2" type="password" autocomplete="new-password" minlength="8" data-match="#new" data-error-match="Пароли не совпадают" required></div>
             <button class="btn btn-primary" type="submit">Сменить пароль</button>
             <a th:if="${currentUser != null and !currentUser.mustChangePassword}"
                class="btn btn-secondary" th:href="${currentUser.admin} ? @{/admin} : @{/student}">Отмена</a>