[gharlan]

Adds ViewResolver::overrideNamespace(), so a project can replace the views of a whole namespace from one directory instead of registering every class separately. The main use case is using the form system in the frontend with a different CSS framework (UIkit, Tailwind, …):

if (Core::isFrontend()) {
    ViewResolver::overrideNamespace('Redaxo\Core\Form', __DIR__ . '/views/form');
}

• The view of a class is looked up by its path relative to the namespace (Redaxo\Core\Form\Widget\Input → <dir>/Widget/Input.view.php), sub-namespaces included.
• Classes without a view file in the directory fall back to their co-located (backend) view. No frontend views are shipped for now.
• Class and namespace overrides share one registry; the last registration wins.
• Overrides stay global. An environment parameter was considered but dropped: the environment is fixed per process, so an if (Core::isFrontend()) around the registration is equivalent and clearer.

Follow-up: the submit/abort buttons are still hard-coded markup in Form and can't be overridden yet.