Send students to a chosen page when registration succeeds

The Student Registration block's "After email confirmation" panel becomes
"After registration": the page it selects is now where a newly registered
student continues to, and a new autoRedirect toggle sends them there
instead of showing the link.

Only the two finished states qualify (RegistrationPage::isRegistrationComplete):
an invited student who is now logged in, and a self-signup back from the
emailed confirmation link. A validation error, an expired confirmation
link, and the intermediate "check your email" step all stay on the page so
their message is read.

The invited-student success previously had no link at all; it gains a
"Continue to your account" one. That path deliberately has no
WordPress-login-screen fallback — pointing someone already signed in at the
login screen helps nobody — so continueUrl() distinguishes "no page chosen"
from "page chosen", and the redirect does nothing until one is picked.

Closes #115

Co-Authored-By: Claude Opus 5 <[email protected]>

templates/frontend/register-page.php

@@ -14,7 +14,7 @@ if (! defined('ABSPATH')) {
  * @var bool   $canRegister
  * @var string $inviteOnlyMessage Text shown when registration is closed and no valid invite is present.
  * @var bool   $open   Whether open (self-approval) registration is enabled.
- * @var string $successType '' | 'invite' (created + logged in) | 'confirm' (check email) | 'confirm_group' (check email; auto-approved on confirm).
+ * @var string $successType '' | 'confirm' (check email) | 'confirm_group' (check email; auto-approved on confirm). The invited-student success is rendered by RegistrationPage::render() itself, which returns before this template for logged-in visitors.
  * @var string $confirmResult '' | '1' (email confirmed, awaiting approval) | 'ready' (confirmed + auto-approved) | 'expired'.
  * @var string $loginUrl Where the post-confirmation sign-in link points.
  * @var string $error
@@ -57,9 +57,7 @@ $renderQuestionField = static function (Question $question): void {
 };
 ?>
 <div class="us-register-form">
-    <?php if ($successType === 'invite') : ?>
-        <p class="us-success"><?php esc_html_e('Your account has been created and you are now logged in.', 'unsupervised-schedular'); ?></p>
-    <?php elseif ($successType === 'confirm') : ?>
+    <?php if ($successType === 'confirm') : ?>
         <p class="us-success"><?php esc_html_e('Your account has been created. Check your email for a link to confirm your address — once you do, a studio admin will review and approve your account.', 'unsupervised-schedular'); ?></p>
     <?php elseif ($successType === 'confirm_group') : ?>
         <p class="us-success"><?php esc_html_e('Your account has been created. Check your email for a link to confirm your address — once you do, your account is ready to use.', 'unsupervised-schedular'); ?></p>