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>

.claude/launch.json

@@ -25,9 +25,9 @@
       "runtimeArgs": [
         "-NoProfile",
         "-Command",
-        "$env:APP_ENV='dev'; $env:DATABASE_URL='sqlite:///C:/Django Project/KantSites/var/browser-tests/pilot-539a85bd3015/pilot.db'; $env:MEDIA_DIRECTORY='C:/Django Project/KantSites/var/kantit-media'; & C:/php84/php.exe -d upload_max_filesize=8M -d post_max_size=10M -d memory_limit=256M -S 127.0.0.1:8087 -t public bin/dev-router.php"
+        "$env:APP_ENV='dev'; $env:DATABASE_URL='sqlite:///C:/Django Project/KantSites/var/browser-tests/pilot-1d19ca1bb9ca/pilot.db'; $env:MEDIA_DIRECTORY='C:/Django Project/KantSites/var/kantit-media'; & C:/php84/php.exe -d upload_max_filesize=8M -d post_max_size=10M -d memory_limit=256M -S 127.0.0.1:8091 -t public bin/dev-router.php"
       ],
-      "port": 8087
+      "port": 8091
     },
     {
       "name": "elektro-bernhard",