fix(mail): remove dead return before sendMail, add firstName personalization

- Remove I18n dependency injection from MailService constructor
- Hardcode German email subject/body strings directly in the service
- Add optional firstName field to userSignUp and forgotPassword methods
- Wire firstName from user object through auth.service.ts call sites
- Add comprehensive unit tests for MailService

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

myteamwallet_backend/src/auth/auth.service.ts

@@ -192,6 +192,7 @@ export class AuthService {
       to: user.email,
       data: {
         hash,
+        firstName: user.firstName,
       },
     });
   }
@@ -247,6 +248,7 @@ export class AuthService {
         to: email,
         data: {
           hash,
+          firstName: user.firstName,
         },
       });
     }