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/Content/FormRepository.php

@@ -164,6 +164,11 @@ final class FormRepository
         if (false === filter_var($recipient, FILTER_VALIDATE_EMAIL)) {
             throw new InvalidForm('Bitte gib eine gültige Empfängeradresse an.');
         }
+        // Der Absender ist freiwillig: leer heisst, die Post kommt von der Empfaengeradresse.
+        $sender = trim((string) ($settings['sender'] ?? ''));
+        if ('' !== $sender && false === filter_var($sender, FILTER_VALIDATE_EMAIL)) {
+            throw new InvalidForm('Bitte gib eine gültige Absenderadresse an oder lass das Feld leer.');
+        }
         $replyTo = trim((string) ($settings['replyToField'] ?? ''));
         if ('' !== $replyTo) {
             $match = array_filter($fields, static fn (array $f): bool => $f['name'] === $replyTo && 'email' === $f['type']);
@@ -174,6 +179,7 @@ final class FormRepository
 
         $clean = [
             'recipient' => $recipient,
+            'sender' => $sender,
             'subject' => mb_substr(trim((string) ($settings['subject'] ?? '')), 0, 160) ?: 'Neue Formularnachricht',
             'success' => mb_substr(trim((string) ($settings['success'] ?? '')), 0, 300) ?: 'Vielen Dank. Deine Nachricht ist angekommen.',
             'replyToField' => $replyTo,