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_guild_test.go

@@ -7,9 +7,15 @@ import (
 	"go.uber.org/zap"
 )
 
+func newTestMailService(mr MailRepo, gr GuildRepo) *MailService {
+	logger, _ := zap.NewDevelopment()
+	return NewMailService(mr, gr, logger)
+}
+
 func newTestGuildService(gr GuildRepo, mr MailRepo) *GuildService {
 	logger, _ := zap.NewDevelopment()
-	return NewGuildService(gr, mr, nil, logger)
+	ms := newTestMailService(mr, gr)
+	return NewGuildService(gr, ms, nil, logger)
 }
 
 func TestGuildService_OperateMember(t *testing.T) {