- Merge lang package into constants (add IsValidLang, ValidateLang, AllLangs)
- Rename internal/services to internal/email for consistency with pdf package
- Rename types to avoid redundancy: EmailService→Service, EmailConfig→Config
- Update all imports and references across codebase
- Delete internal/lang directory (functions moved to constants)
- Add EmailSender interface to allow mocking in tests
- Add IsInitialized() method to template.Manager for nil-safe checks
- Update contact handler to use interface and safe initialization checks
- Add mockEmailSender in security tests to avoid SMTP connection attempts
- Use unique IPs per test case to avoid rate limiting interference