Show constraint validation messages when form submission fails

Problem

Submitting an invalid form in Photon focuses the first invalid control but does not show a validation message. In Safari, Chrome, and Firefox, submitting the test email field with a value such as sdd displays a browser-provided message explaining that the address is missing an @.

Photon's Engine implements constraint checking and reportValidity() focusing, but currently does not report the constraint problem to the user. The existing implementation leaves this as a FIXME in Engine/Libraries/LibWeb/HTML/FormAssociatedElement.cpp (report_validity_steps).

Reproduction

1. Open Documentation/ManualTesting/CoreBrowser/input-forms.html.
2. Enter sdd in the required email field.
3. Submit the form.

Expected

Photon shows an accessible, localized validation message for the first invalid control and keeps submission canceled. The message should update or dismiss as the user corrects the value or moves focus, following browser behavior.

Actual

The invalid control is focused, but no validation message is shown.

Context

The result was compared with another browser on macOS using the core browser manual test suite.