chore: add php-cs-fixer with PSR-12 config and CI check

- Add friendsofphp/php-cs-fixer to require-dev
- Create .php-cs-fixer.dist.php configured for @PSR12 ruleset
- Add php-cs-fixer dry-run step to CI pipeline
- Auto-fix existing PSR-12 violations
- Document code style tooling in readme.md

src/Service/LoginInterface.php

@@ -7,7 +7,8 @@ use Psr\Cache\InvalidArgumentException;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 
-interface LoginInterface {
+interface LoginInterface
+{
     /** @throws InvalidArgumentException */
     public function checkToken(Payload $payload, Request $request): ?Response;
 }