chore(mail): remove nestjs-i18n, only ever used for the two mail templates

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

myteamwallet_backend/src/config/app.config.ts

@@ -8,6 +8,4 @@ export default registerAs('app', () => ({
   backendDomain: process.env.BACKEND_DOMAIN,
   port: parseInt(process.env.APP_PORT || process.env.PORT, 10) || 3000,
   apiPrefix: process.env.API_PREFIX || 'api',
-  fallbackLanguage: process.env.APP_FALLBACK_LANGUAGE || 'en',
-  headerLanguage: process.env.APP_HEADER_LANGUAGE || 'x-custom-lang',
 }));