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/model_character.go

@@ -155,7 +155,7 @@ func (save *CharacterSaveData) updateSaveDataWithStruct() {
 
 // This will update the save struct with the values stored in the character save
 func (save *CharacterSaveData) updateStructWithSaveData() {
-	save.Name = stringsupport.SJISToUTF8(bfutil.UpToNull(save.decompSave[88:100]))
+	save.Name, _ = stringsupport.SJISToUTF8(bfutil.UpToNull(save.decompSave[88:100]))
 	if save.decompSave[save.Pointers[pGender]] == 1 {
 		save.Gender = true
 	} else {