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/signserver/session.go

@@ -157,7 +157,8 @@ func (s *Session) handlePSSGN(bf *byteframe.ByteFrame) {
 
 func (s *Session) handlePSNLink(bf *byteframe.ByteFrame) {
 	_ = bf.ReadNullTerminatedBytes() // Client ID
-	credentials := strings.Split(stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes()), "\n")
+	credStr, _ := stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
+	credentials := strings.Split(credStr, "\n")
 	token := string(bf.ReadNullTerminatedBytes())
 	uid, resp := s.server.validateLogin(credentials[0], credentials[1])
 	if resp == SIGN_SUCCESS && uid > 0 {
@@ -199,8 +200,8 @@ func (s *Session) handlePSNLink(bf *byteframe.ByteFrame) {
 }
 
 func (s *Session) handleDSGN(bf *byteframe.ByteFrame) {
-	user := stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
-	pass := stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
+	user, _ := stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
+	pass, _ := stringsupport.SJISToUTF8(bf.ReadNullTerminatedBytes())
 	_ = string(bf.ReadNullTerminatedBytes()) // Unk
 	s.authenticate(user, pass)
 }

server/signserver/sign_server.go

@@ -76,7 +76,8 @@ func (s *Server) acceptClients() {
 			if shutdown {
 				break
 			} else {
-				panic(err)
+				s.logger.Warn("Error accepting client", zap.Error(err))
+				continue
 			}
 		}