feat: add mx login mode selection

exec/lib/Settings.php

@@ -58,6 +58,7 @@ final class Settings
         return [
             'DEFAULT_ENABLE' => 'true',
             'MAIL_LOGIN_METHOD' => 'login_url',
+            'MX_LOGIN_MODE' => '1',
             'MAIL_LOGIN_TARGET_NAME' => 'mx1',
             'MAIL_LOGIN_TARGET_BASE_URL' => 'https://mx1.mojserwer.pl:2222',
             'MAIL_LOGIN_REDIRECT_URL' => '/',
@@ -115,6 +116,11 @@ final class Settings
         return $this->getString('MAIL_LOGIN_METHOD', 'login_url');
     }
 
+    public function mxLoginMode(): int
+    {
+        return (int)$this->getString('MX_LOGIN_MODE', '1');
+    }
+
     public function redirectUrl(): string
     {
         return $this->getString('MAIL_LOGIN_REDIRECT_URL', '/');
@@ -275,6 +281,9 @@ final class Settings
         if ($this->method() !== 'login_url') {
             throw new InvalidArgumentException('MAIL_LOGIN_METHOD must be login_url');
         }
+        if (!in_array($this->getString('MX_LOGIN_MODE', '1'), ['1', '2'], true)) {
+            throw new InvalidArgumentException('MX_LOGIN_MODE must be 1 or 2');
+        }
         if (!in_array($this->clientIpMode(), ['direct', 'trusted_proxy'], true)) {
             throw new InvalidArgumentException('MAIL_LOGIN_CLIENT_IP_MODE must be direct or trusted_proxy');
         }