refactor: consolidate lang into constants, rename services to email

- 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)

internal/config/config.go

@@ -56,12 +56,12 @@ func Load() *Config {
 			WriteTimeout: getEnvAsInt("WRITE_TIMEOUT", 15),
 		},
 		Template: TemplateConfig{
-			Dir:         getEnv("TEMPLATE_DIR", "templates"),
-			PartialsDir: getEnv("PARTIALS_DIR", "templates/partials"),
+			Dir:         getEnv("TEMPLATE_DIR", c.DirTemplates),
+			PartialsDir: getEnv("PARTIALS_DIR", c.DirPartials),
 			HotReload:   getEnvAsBool("TEMPLATE_HOT_RELOAD", isDevelopment()),
 		},
 		Data: DataConfig{
-			Dir: getEnv("DATA_DIR", "data"),
+			Dir: getEnv("DATA_DIR", c.DirData),
 		},
 		Email: EmailConfig{
 			SMTPHost:     getEnv("SMTP_HOST", "smtp.gmail.com"),