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/handlers_mail_test.go

@@ -386,6 +386,7 @@ func TestHandleMsgMhfSendMail_Direct(t *testing.T) {
 	server := createMockServer()
 	mock := &mockMailRepo{}
 	server.mailRepo = mock
+	ensureMailService(server)
 	session := createMockSession(1, server)
 
 	pkt := &mhfpacket.MsgMhfSendMail{
@@ -436,6 +437,7 @@ func TestHandleMsgMhfSendMail_Guild(t *testing.T) {
 	}
 	server.mailRepo = mailMock
 	server.guildRepo = guildMock
+	ensureMailService(server)
 	session := createMockSession(1, server)
 
 	pkt := &mhfpacket.MsgMhfSendMail{
@@ -470,6 +472,7 @@ func TestHandleMsgMhfSendMail_GuildNotFound(t *testing.T) {
 	guildMock := &mockGuildRepoForMail{getErr: errNotFound}
 	server.mailRepo = mailMock
 	server.guildRepo = guildMock
+	ensureMailService(server)
 	session := createMockSession(1, server)
 
 	pkt := &mhfpacket.MsgMhfSendMail{