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/student/lab.html

@@ -93,6 +93,7 @@
                 <input type="hidden" name="_csrf" th:value="${csrfToken}">
                 <input type="hidden" name="labId" th:value="${lab.id}">
                 <input class="input w-260" name="code" placeholder="6-значный код" maxlength="6" pattern="[0-9]{6}"
+                       data-error-pattern="Введите 6 цифр"
                        inputmode="numeric" autocomplete="one-time-code" required>
                 <button class="btn btn-primary" type="submit">Приступить</button>
             </form>