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_rengoku.go

@@ -103,7 +103,9 @@ func handleMsgMhfGetRengokuBinary(s *Session, p mhfpacket.MHFPacket) {
 	// a (massively out of date) version resides in the game's /dat/ folder or up to date can be pulled from packets
 	data, err := os.ReadFile(filepath.Join(s.server.erupeConfig.BinPath, "rengoku_data.bin"))
 	if err != nil {
-		panic(err)
+		s.logger.Error("Failed to read rengoku_data.bin", zap.Error(err))
+		doAckBufFail(s, pkt.AckHandle, nil)
+		return
 	}
 	doAckBufSucceed(s, pkt.AckHandle, data)
 }