feat: upgrade verification code email with bilingual HTML template

- Chinese domains (qq.com, 163.com, etc.) receive a Chinese email
- All other domains receive an English email
- Prominent code display: 40px monospace with wide letter-spacing
- Clean OpenAI-inspired layout with dark header and card design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/service/user_service.go

@@ -130,8 +130,7 @@ func (svc *UserService) SendEmailVerifyCode(ctx context.Context, emailAddr strin
 
 	code := fmt.Sprintf("%06d", rand.Intn(1000000))
 
-	subject := "Your verification code"
-	body := fmt.Sprintf("Your verification code is: %s\nIt will expire in 10 minutes.", code)
+	subject, body := email.BuildVerifyCodeEmail(emailAddr, code)
 	if err := email.Send(ctx, emailAddr, subject, body); err != nil {
 		log.Error(ctx, "func", "SendEmailVerifyCode", "msg", "send email error", "error", err)
 		return err