Fix Paystack Fullscreen permission policy for Inline iframes.

Pre-set allow=fullscreen on Paystack iframes before navigation (createElement
hook + MutationObserver on allow/src/id), re-apply if Paystack overwrites
allow, and send Permissions-Policy HTTP headers so checkout.paystack.com can
use fullscreen inside the in-app sheet.

tests/Feature/ResponsivePaystackSheetTest.php

@@ -33,6 +33,11 @@ class ResponsivePaystackSheetTest extends TestCase
         $this->assertStringContainsString('Complete payment on this screen', $html);
         $this->assertStringContainsString('not in a separate browser', $html);
         $this->assertStringContainsString('items-end justify-center md:items-center', $html);
+        // Cross-origin checkout needs fullscreen delegated on the iframe + parent policy.
+        $this->assertStringContainsString('ensurePaystackIframePermissions', $html);
+        $this->assertStringContainsString('installIframeAllowHook', $html);
+        $this->assertStringContainsString('fullscreen *', $html);
+        $this->assertStringContainsString("attributeFilter: ['allow', 'src', 'id', 'name']", $html);
         // Must not push Paystack to a separate browser as the primary path.
         $this->assertStringNotContainsString('Continue to Paystack', $html);
         $this->assertStringNotContainsString('window.open(', $html);