Add redirectAfterLogin() helper

[dereuromark]

Summary

This adds a small controller-level helper to make the safe post-login redirect path the obvious one:

return $this->Authentication->redirectAfterLogin('/dashboard');

Internally this builds on the existing safe redirect parsing already present in the plugin via AuthenticationService::getLoginRedirect() and AuthenticationComponent::getLoginRedirect().

Why

The plugin already does the hard part correctly:

• AuthenticationService::getLoginRedirect() rejects redirect targets with a scheme or host
• it normalizes local paths
• it applies optional redirect-loop validation

The gap is ergonomics.

In real applications, controller code often ends up doing this instead:

$redirect = $this->request->getQuery('redirect', '/dashboard');
return $this->redirect($redirect);

or this:

$redirect = $this->Authentication->getLoginRedirect('/dashboard') ?? '/dashboard';
return $this->redirect($redirect);

The second version is safe, but it is a two-step pattern that is easy to bypass or forget. A first-class helper makes the secure path shorter and more discoverable.

What this adds

This PR adds:

AuthenticationComponent::redirectAfterLogin(array|string|null $default = '/')

Behavior:

1. Resolve the validated local redirect target using existing logic: getLoginRedirect($default)
2. Fallback to $default when no valid redirect is available
3. Return the controller redirect response directly

Example:

if ($result->isValid()) {
    return $this->Authentication->redirectAfterLogin('/dashboard');
}

We could release this as a new minor.

[dereuromark]

Yeah, sounds right.
I adjusted it.