fix(channelserver): add fail ACKs to silent error paths to prevent client softlocks

Handlers that log errors and return without sending a MsgSysAck leave
the client waiting indefinitely. Add doAckSimpleFail/doAckBufFail to
14 error paths across 4 files, matching the pattern already used in
~70 other error paths across the codebase.

Affected handlers:
- handleMsgMhfGetCafeDuration (1 path)
- handleMsgMhfSavedata (1 path)
- handleMsgMhfArrangeGuildMember (3 paths)
- handleMsgMhfEnumerateGuildMember (5 paths)
- handleMsgSysLogin (4 paths)
- handleMsgSysIssueLogkey (1 path)

server/channelserver/handlers_cafe.go

@@ -97,6 +97,7 @@ func handleMsgMhfGetCafeDuration(s *Session, p mhfpacket.MHFPacket) {
 	err = s.server.db.QueryRow("SELECT cafe_time FROM characters WHERE id = $1", s.charID).Scan(&cafeTime)
 	if err != nil {
 		s.logger.Error("Failed to get cafe time", zap.Error(err))
+		doAckBufFail(s, pkt.AckHandle, make([]byte, 4))
 		return
 	}
 	if mhfcourse.CourseExists(30, s.courses) {