refactor(mail): extract mail logic into MailService

Introduce MailService as a convenience layer between handlers/services
and MailRepo. Provides Send, SendSystem, SendGuildInvite, and
BroadcastToGuild methods that encapsulate the boolean flag combinations.

GuildService now depends on MailService instead of MailRepo directly,
simplifying its mail-sending calls from verbose SendMail(..., false, true)
to clean SendSystem(recipientID, subject, body).

Guild mail broadcast logic moved from handleMsgMhfSendMail into
MailService.BroadcastToGuild.

server/channelserver/svc_mail.go

@@ -0,0 +1,55 @@
+package channelserver
+
+import (
+	"fmt"
+
+	"go.uber.org/zap"
+)
+
+// MailService encapsulates mail-sending business logic, sitting between
+// handlers/services and the MailRepo. It provides convenient methods for
+// common mail patterns (system notifications, guild broadcasts, player mail)
+// so callers don't need to specify boolean flags directly.
+type MailService struct {
+	mailRepo  MailRepo
+	guildRepo GuildRepo
+	logger    *zap.Logger
+}
+
+// NewMailService creates a new MailService.
+func NewMailService(mr MailRepo, gr GuildRepo, log *zap.Logger) *MailService {
+	return &MailService{
+		mailRepo:  mr,
+		guildRepo: gr,
+		logger:    log,
+	}
+}
+
+// Send sends a player-to-player mail with an optional item attachment.
+func (svc *MailService) Send(senderID, recipientID uint32, subject, body string, itemID, quantity uint16) error {
+	return svc.mailRepo.SendMail(senderID, recipientID, subject, body, itemID, quantity, false, false)
+}
+
+// SendSystem sends a system notification mail (no item, flagged as system message).
+func (svc *MailService) SendSystem(recipientID uint32, subject, body string) error {
+	return svc.mailRepo.SendMail(0, recipientID, subject, body, 0, 0, false, true)
+}
+
+// SendGuildInvite sends a guild invitation mail (flagged as guild invite).
+func (svc *MailService) SendGuildInvite(senderID, recipientID uint32, subject, body string) error {
+	return svc.mailRepo.SendMail(senderID, recipientID, subject, body, 0, 0, true, false)
+}
+
+// BroadcastToGuild sends a mail from senderID to all members of the specified guild.
+func (svc *MailService) BroadcastToGuild(senderID, guildID uint32, subject, body string) error {
+	members, err := svc.guildRepo.GetMembers(guildID, false)
+	if err != nil {
+		return fmt.Errorf("get guild members for broadcast: %w", err)
+	}
+	for _, m := range members {
+		if err := svc.mailRepo.SendMail(senderID, m.CharID, subject, body, 0, 0, false, false); err != nil {
+			return fmt.Errorf("send guild broadcast to char %d: %w", m.CharID, err)
+		}
+	}
+	return nil
+}