Link a signed-in visitor to the configured continue page

The registration page's already-logged-in branch returned a bare
sentence with nowhere to go, leaving the visitor to find their own way
to their account. The invited-student branch a few lines above already
built exactly the link that was missing.

Extract that into continueLink() and use it for both logged-in
outcomes. There is deliberately still no wp_login_url() fallback:
sending someone already signed in to the login screen is the same dead
end with extra steps, so with no page configured there is no link.

Both messages now carry the us-register-form wrapper and enqueue the
plugin stylesheet, which the invite branch emitted markup for but never
loaded.

Closes #131

tests/Unit/Auth/RegistrationPageTest.php

@@ -504,6 +504,37 @@ class RegistrationPageTest extends TestCase
         self::assertStringContainsString('href="http://home.test/welcome/"', $html);
     }
 
+    public function testAlreadyLoggedInVisitorIsLinkedToTheChosenPage(): void
+    {
+        // No us_registered flag: someone who simply happens to be signed in and
+        // lands on the registration page. They still need a way onward.
+        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('already have an account', $html);
+        self::assertStringContainsString('href="http://home.test/welcome/"', $html);
+        // Not the just-registered message — that branch needs its own flag.
+        self::assertStringNotContainsString('us-success', $html);
+    }
+
+    public function testAlreadyLoggedInVisitorGetsNoLinkWithoutAChosenPage(): void
+    {
+        Functions\when('is_user_logged_in')->justReturn(true);
+        Functions\when('sanitize_key')->alias(static fn ($v) => strtolower((string) $v));
+        Functions\when('get_permalink')->justReturn(false);
+
+        // A deleted page resolves to false, which must not become a broken link.
+        $html = $this->ctx['page']->render([ 'loginPageId' => 4 ]);
+
+        self::assertStringContainsString('already have an account', $html);
+        self::assertStringNotContainsString('<a href', $html);
+
+        self::assertStringNotContainsString('<a href', $this->ctx['page']->render([]));
+    }
+
     public function testContinueUrlIsNullWithoutAResolvablePage(): void
     {
         Functions\when('get_permalink')->justReturn(false);