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_guild_alliance.go

@@ -195,8 +195,8 @@ func handleMsgMhfOperateJoint(s *Session, p mhfpacket.MHFPacket) {
 			doAckSimpleFail(s, pkt.AckHandle, make([]byte, 4))
 		}
 	default:
+		s.logger.Error("unhandled operate joint action", zap.Uint8("action", uint8(pkt.Action)))
 		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
-		panic(fmt.Sprintf("Unhandled operate joint action '%d'", pkt.Action))
 	}
 }