fix(protbot,channelserver): fix sign protocol and entrance parsing, guard nil save data

The protbot sent "DSGN:\x00" as the sign request type, but the server
strips the last 3 characters as a version suffix. Send "DSGN:041"
(ZZ client mode 41) to match the real client format.

The entrance channel entry parser read 14 bytes for remaining fields
but the server writes 18 bytes (9 uint16, not 7), causing a panic
when parsing the server list.

The channel server panicked on disconnect when a session had no
decompressed save data (e.g. protbot or early client disconnect).
Guard Save() against nil decompSave.

Also fix docker-compose volume mount for Postgres 18 which changed
its data directory layout.

server/channelserver/handlers_character.go

@@ -48,6 +48,13 @@ func GetCharacterSaveData(s *Session, charID uint32) (*CharacterSaveData, error)
 }
 
 func (save *CharacterSaveData) Save(s *Session) {
+	if save.decompSave == nil {
+		s.logger.Warn("No decompressed save data, skipping save",
+			zap.Uint32("charID", save.CharID),
+		)
+		return
+	}
+
 	if !s.kqfOverride {
 		s.kqf = save.KQF
 	} else {