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.

network/mhfpacket/msg_mhf_send_mail.go

@@ -35,8 +35,8 @@ func (m *MsgMhfSendMail) Parse(bf *byteframe.ByteFrame, ctx *clientctx.ClientCon
 	bf.ReadUint16() // Zeroed
 	m.Quantity = bf.ReadUint16()
 	m.ItemID = bf.ReadUint16()
-	m.Subject = stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
-	m.Body = stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
+	m.Subject, _ = stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
+	m.Body, _ = stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
 	return nil
 }