Give mail delivery its own admin page and move the origin probe off /.well-known

The mail page under System tries SMTP, PHP mail() and both sendmail
modes one by one or all at once, and saves the chosen way to .env
without ever showing the password. IONOS answers everything under
/.well-known/ itself with 403, so the address and HTTPS checks never
reached the installation; the probe now lives at /kantsites-origin
(ADR 122).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/HttpsRedirectTest.php

@@ -18,7 +18,7 @@ final class HttpsRedirectTest extends TestCase
     /** @var list<?string> */
     private array $answers = [];
     /** @var array{status: int, location: string, error: string} */
-    private array $http = ['status' => 301, 'location' => 'https://www.example.de/.well-known/kantsites-origin', 'error' => ''];
+    private array $http = ['status' => 301, 'location' => 'https://www.example.de/kantsites-origin', 'error' => ''];
 
     protected function setUp(): void
     {