Let a form send from its own address in 1.15.1

Umut asked where a contact form's mail is sent from: it was the recipient
address, `From` equal to `To`. That breaks as soon as the recipient is a
mailbox at another provider, because most hosts only deliver mail that
claims an address of their own domain, and the provider's SPF/DMARC then
treats it as forged.

Each form now has an optional `sender` setting, validated like the
recipient; empty keeps the old behaviour, so stored forms need no
migration. Delivery uses `sender ?: recipient`; the reply-to field is
untouched. The administration shows the field under the recipient with a
note on why it matters. The pilot launch config moves to port 8091 because
8087 was held by another session.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/Controller/FormAdminController.php

@@ -62,6 +62,7 @@ final class FormAdminController extends AbstractController
                 $fields,
                 [
                     'recipient' => $request->request->getString('recipient'),
+                    'sender' => $request->request->getString('sender'),
                     'subject' => $request->request->getString('subject'),
                     'success' => $request->request->getString('success'),
                     'replyToField' => $request->request->getString('replyToField'),