feat(mail): Add support for multiple domains per mail service

- Add `Domains` field to MailServiceConfig for managing additional domains under single API
- Implement `matchDomain` helper function for precise and subdomain matching logic
- Update `GetServiceByDomain` to check both primary domain and additional domains list
- Enhance EmailConfig UI to display domain count and allow comma-separated domain input
- Add domains field to mail service request/response structures in API handlers
- Update frontend types to include domains array in MailService interface
- Improve documentation with clarification on primary vs additional domains usage
- Allows single mail service API to manage multiple email domains for verification and operations

backend/internal/mail/service.go

@@ -100,14 +100,31 @@ func GetRandomService() config.MailServiceConfig {
 	return currentMailServices[rand.Intn(len(currentMailServices))]
 }
 
+// matchDomain 检查邮箱域名是否匹配服务域名（精确匹配或子域名匹配）
+func matchDomain(emailDomain, serviceDomain string) bool {
+	if serviceDomain == "" {
+		return false
+	}
+	return emailDomain == serviceDomain || strings.HasSuffix(emailDomain, "."+serviceDomain)
+}
+
 // GetServiceByDomain 根据域名获取对应的邮箱服务
+// 会同时检查 Domain（主域名）和 Domains（附加域名列表）
 func GetServiceByDomain(domain string) *config.MailServiceConfig {
 	mailServicesMutex.RLock()
 	defer mailServicesMutex.RUnlock()
 
-	for _, s := range currentMailServices {
-		if s.Domain == domain || strings.HasSuffix(domain, "."+s.Domain) {
-			return &s
+	for i := range currentMailServices {
+		s := &currentMailServices[i]
+		// 检查主域名
+		if matchDomain(domain, s.Domain) {
+			return s
+		}
+		// 检查附加域名列表
+		for _, d := range s.Domains {
+			if matchDomain(domain, d) {
+				return s
+			}
 		}
 	}
 	return nil