Show a sign-in link instead of the form after email confirmation

Closes #67

When a student lands on the registration page from the confirmation
email (?us_confirmed=1), replace the registration form with the
confirmation message and a "Sign in to your account" link — the form
is useless at that point and re-submitting would only produce an
"account already exists" error. A confirmed-but-unapproved student can
already log in (the pending gate only withholds booking), so signing in
is the natural next step.

The link target follows the booking block's pattern: a loginPageId
block attribute (page picker in the editor sidebar) or login_page_id
shortcode attribute, falling back to wp_login_url(). The expired-link
notice keeps the form as before.

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

tests/Unit/Auth/RegistrationPageTest.php

@@ -57,9 +57,20 @@ class RegistrationPageTest extends TestCase
     protected function tearDown(): void
     {
         $_POST = [];
+        $_GET  = [];
         parent::tearDown();
     }
 
+    /** Stub everything render() needs on a logged-out GET request. */
+    private function stubRenderContext(): void
+    {
+        Functions\when('is_user_logged_in')->justReturn(false);
+        Functions\when('sanitize_key')->alias(static fn ($v) => strtolower((string) $v));
+        Functions\when('wp_login_url')->justReturn('http://home.test/wp-login.php');
+        Functions\when('wp_nonce_field')->justReturn('');
+        $this->ctx['settings']->shouldReceive('openRegistrationEnabled')->andReturn(true);
+    }
+
     private function submit(?Invite $invite, bool $open): string
     {
         $method = new \ReflectionMethod(RegistrationPage::class, 'handleSubmit');
@@ -120,6 +131,51 @@ class RegistrationPageTest extends TestCase
         self::assertNotSame('', $result);
     }
 
+    public function testConfirmedEmailShowsSignInLinkInsteadOfForm(): void
+    {
+        $_GET = ['us_confirmed' => '1'];
+        $this->stubRenderContext();
+
+        $html = $this->ctx['page']->render([]);
+
+        self::assertStringContainsString('us-success', $html);
+        self::assertStringContainsString('http://home.test/wp-login.php', $html);
+        self::assertStringNotContainsString('<form', $html);
+    }
+
+    public function testConfirmedSignInLinkUsesConfiguredLoginPage(): void
+    {
+        $_GET = ['us_confirmed' => '1'];
+        $this->stubRenderContext();
+        Functions\expect('get_permalink')->once()->with(7)->andReturn('http://home.test/sign-in/');
+
+        $html = $this->ctx['page']->render(['login_page_id' => 7]);
+
+        self::assertStringContainsString('http://home.test/sign-in/', $html);
+        self::assertStringNotContainsString('wp-login.php', $html);
+    }
+
+    public function testRegistrationFormRendersWithoutConfirmationFlag(): void
+    {
+        $this->stubRenderContext();
+
+        $html = $this->ctx['page']->render([]);
+
+        self::assertStringContainsString('<form', $html);
+        self::assertStringNotContainsString('us-success', $html);
+    }
+
+    public function testExpiredConfirmationStillShowsForm(): void
+    {
+        $_GET = ['us_confirmed' => 'expired'];
+        $this->stubRenderContext();
+
+        $html = $this->ctx['page']->render([]);
+
+        self::assertStringContainsString('us-error', $html);
+        self::assertStringContainsString('<form', $html);
+    }
+
     public function testRejectsWhenARequiredPolicyIsUnaccepted(): void
     {
         $_POST = [ 'password' => 'password123', 'display_name' => 'Ada', 'email' => '[email protected]' ];

tests/Unit/BlockRegistrarTest.php

@@ -125,7 +125,10 @@ class BlockRegistrarTest extends TestCase
             ['bookingPageId', 'autoRedirect'],
             array_keys($registered['us-scheduler/student-login']['attributes'])
         );
-        self::assertSame([], $registered['us-scheduler/student-register']['attributes']);
+        self::assertSame(
+            ['loginPageId'],
+            array_keys($registered['us-scheduler/student-register']['attributes'])
+        );
         self::assertSame(
             ['offeringId'],
             array_keys($registered['us-scheduler/group-classes']['attributes'])