refactor: replace panic calls with structured error handling

Replace ~25 panic() calls in non-fatal code paths with proper
s.logger.Error + return patterns. Panics in handler code crashed
goroutines (caught by defer/recover but still disruptive) instead
of failing gracefully.

Key changes:
- SJISToUTF8 now returns (string, error); all 30+ callers updated
- Handler DB/IO panics replaced with log + return/ack fail
- Unhandled switch-case panics replaced with logger.Error
- Sign server Accept() panic replaced with log + continue
- Dead unreachable panic in guild_model.go removed
- deltacomp patch error logs and returns partial data

Panics intentionally kept: ByteFrame sentinel, unimplemented
packet stubs, os.Exit in main.go.

server/channelserver/handlers_festa.go

@@ -17,15 +17,7 @@ import (
 
 func handleMsgMhfSaveMezfesData(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfSaveMezfesData)
-	if len(pkt.RawDataPayload) > 4096 {
-		s.logger.Warn("MezFes payload too large", zap.Int("len", len(pkt.RawDataPayload)))
-		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
-		return
-	}
-	if _, err := s.server.db.Exec(`UPDATE characters SET mezfes=$1 WHERE id=$2`, pkt.RawDataPayload, s.charID); err != nil {
-		s.logger.Error("Failed to save mezfes data", zap.Error(err))
-	}
-	doAckSimpleSucceed(s, pkt.AckHandle, []byte{0x00, 0x00, 0x00, 0x00})
+	saveCharacterData(s, pkt.AckHandle, "mezfes", pkt.RawDataPayload, 4096)
 }
 
 func handleMsgMhfLoadMezfesData(s *Session, p mhfpacket.MHFPacket) {