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.

cmd/protbot/scenario/chat.go

@@ -62,8 +62,8 @@ func ListenChat(ch *protocol.ChannelConn, cb ChatCallback) {
 		_ = pbf.ReadUint16() // flags
 		_ = pbf.ReadUint16() // senderNameLen
 		_ = pbf.ReadUint16() // messageLen
-		msg := stringsupport.SJISToUTF8(pbf.ReadNullTerminatedBytes())
-		sender := stringsupport.SJISToUTF8(pbf.ReadNullTerminatedBytes())
+		msg, _ := stringsupport.SJISToUTF8(pbf.ReadNullTerminatedBytes())
+		sender, _ := stringsupport.SJISToUTF8(pbf.ReadNullTerminatedBytes())
 
 		cb(ChatMessage{
 			ChatType:   chatType,