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]>

src/BlockRegistrar.php

@@ -109,6 +109,7 @@ class BlockRegistrar {
 						'type'    => 'number',
 						'default' => 0,
 					],
+					'autoRedirect'      => $redirectToggle,
 					'inviteOnlyMessage' => [
 						'type'    => 'string',
 						'default' => '',
@@ -170,10 +171,11 @@ class BlockRegistrar {
 	/**
 	 * Server-side auto-redirect for blocks that opt in via their autoRedirect
 	 * attribute: logged-out visitors on a page containing the booking block
-	 * are sent to its login page, and logged-in visitors on a page containing
-	 * the student-login block are sent to its booking page. Hooked on
-	 * `template_redirect` because block rendering happens after output has
-	 * started, too late to send a Location header.
+	 * are sent to its login page, logged-in visitors on a page containing the
+	 * student-login block are sent to its booking page, and a student who has
+	 * just finished registering is sent to the register block's chosen page.
+	 * Hooked on `template_redirect` because block rendering happens after
+	 * output has started, too late to send a Location header.
 	 */
 	public function maybeAutoRedirect(): void {
 		if ( is_admin() || ! is_singular() ) {
@@ -185,6 +187,10 @@ class BlockRegistrar {
 			return;
 		}
 
+		if ( $this->maybeRedirectAfterRegistration( $post ) ) {
+			return;
+		}
+
 		if ( is_user_logged_in() ) {
 			$attrs = $this->firstBlockAttrs( $post->post_content, 'us-scheduler/student-login' );
 			if ( null === $attrs || ! Val::bool( $attrs['autoRedirect'] ?? false ) ) {
@@ -217,6 +223,45 @@ class BlockRegistrar {
 		$this->redirect( $this->bookingPage->loginUrl( $loginPageId ) );
 	}
 
+	/**
+	 * Sends a student whose registration has just completed to the register
+	 * block's chosen page, when the block opts in. Only the finished states
+	 * qualify (see {@see RegistrationPage::isRegistrationComplete()}): a
+	 * failure or the "check your email" step stays put so its message is read.
+	 * Unlike the other blocks there is no login-screen fallback — with no page
+	 * chosen there is nowhere to send them, so the link is shown instead.
+	 *
+	 * Returns whether the redirect was issued (it only ever returns in tests;
+	 * {@see redirect()} exits in production).
+	 */
+	private function maybeRedirectAfterRegistration( \WP_Post $post ): bool {
+		// Checked before parsing the content because it is a couple of query
+		// args, whereas every front-end request would otherwise pay for a
+		// third block scan.
+		if ( ! $this->registrationPage->isRegistrationComplete() ) {
+			return false;
+		}
+
+		$attrs = $this->firstBlockAttrs( $post->post_content, 'us-scheduler/student-register' );
+		if ( null === $attrs || ! Val::bool( $attrs['autoRedirect'] ?? false ) ) {
+			return false;
+		}
+
+		$pageId = Val::int( $attrs['loginPageId'] ?? 0 );
+		if ( $pageId === $post->ID ) {
+			return false; // Redirecting the page to itself would loop.
+		}
+
+		$url = $this->registrationPage->continueUrl( $pageId );
+		if ( null === $url ) {
+			return false;
+		}
+
+		$this->redirect( $url );
+
+		return true;
+	}
+
 	/**
 	 * Attributes of the first occurrence of the named block in the content,
 	 * searching inner blocks so blocks nested inside groups or columns are