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]>

tests/Unit/Auth/RegistrationPageTest.php

@@ -484,6 +484,54 @@ class RegistrationPageTest extends TestCase
 
         self::assertStringContainsString('us-success', $html);
         self::assertStringContainsString('now logged in', $html);
+        // No page chosen: the sign-in-screen fallback is useless to someone who
+        // is already signed in, so no link is offered at all.
+        self::assertStringNotContainsString('<a href', $html);
+    }
+
+    public function testInviteSuccessLinksToTheChosenPage(): void
+    {
+        $_GET = [ 'us_registered' => 'invite' ];
+        Functions\when('is_user_logged_in')->justReturn(true);
+        Functions\when('sanitize_key')->alias(static fn ($v) => strtolower((string) $v));
+        Functions\expect('get_permalink')->once()->with(4)->andReturn('http://home.test/welcome/');
+
+        $html = $this->ctx['page']->render([ 'loginPageId' => 4 ]);
+
+        self::assertStringContainsString('now logged in', $html);
+        self::assertStringContainsString('href="http://home.test/welcome/"', $html);
+    }
+
+    public function testContinueUrlIsNullWithoutAResolvablePage(): void
+    {
+        Functions\when('get_permalink')->justReturn(false);
+
+        self::assertNull($this->ctx['page']->continueUrl(0));
+        self::assertNull($this->ctx['page']->continueUrl(4));
+    }
+
+    public function testIsRegistrationCompleteOnlyForFinishedStates(): void
+    {
+        Functions\when('sanitize_key')->alias(static fn ($v) => strtolower((string) $v));
+
+        // [ query args, logged in, finished ]
+        $cases = [
+            'invited student, now logged in'  => [['us_registered' => 'invite'], true, true],
+            'invited student, not logged in'  => [['us_registered' => 'invite'], false, false],
+            'email confirmed, ready'          => [['us_confirmed' => 'ready'], false, true],
+            'email confirmed, pending review' => [['us_confirmed' => '1'], false, true],
+            'confirmation link expired'       => [['us_confirmed' => 'expired'], false, false],
+            'awaiting email confirmation'     => [['us_registered' => 'confirm'], false, false],
+            'group signup awaiting confirm'   => [['us_registered' => 'confirm_group'], false, false],
+            'plain page view'                 => [[], false, false],
+        ];
+
+        foreach ($cases as $label => [$get, $loggedIn, $expected]) {
+            $_GET = $get;
+            Functions\when('is_user_logged_in')->justReturn($loggedIn);
+
+            self::assertSame($expected, $this->ctx['page']->isRegistrationComplete(), $label);
+        }
     }
 
     public function testInviteOnlyMessageCanBeCustomised(): void