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/compression/deltacomp/deltacomp.go

@@ -92,7 +92,8 @@ func ApplyDataDiff(diff []byte, baseData []byte) []byte {
 		for i := 0; i < differentCount; i++ {
 			b, err := checkReadUint8(patch)
 			if err != nil {
-				panic("Invalid or misunderstood patch format!")
+				zap.L().Error("Invalid or misunderstood patch format", zap.Int("dataOffset", dataOffset))
+				return baseCopy
 			}
 
 			baseCopy[dataOffset+i] = b