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.

common/stringsupport/string_convert.go

@@ -31,13 +31,13 @@ func UTF8ToSJIS(x string) []byte {
 }
 
 // SJISToUTF8 decodes Shift-JIS bytes to a UTF-8 string.
-func SJISToUTF8(b []byte) string {
+func SJISToUTF8(b []byte) (string, error) {
 	d := japanese.ShiftJIS.NewDecoder()
 	result, err := io.ReadAll(transform.NewReader(bytes.NewReader(b), d))
 	if err != nil {
-		panic(err)
+		return "", fmt.Errorf("ShiftJIS decode: %w", err)
 	}
-	return string(result)
+	return string(result), nil
 }
 
 // ToNGWord converts a UTF-8 string into a slice of uint16 values in the