clash api: download clash-dashboard if external-ui directory is empty

common/badversion/version_json.go

@@ -0,0 +1,17 @@
+package badversion
+
+import "github.com/sagernet/sing-box/common/json"
+
+func (v Version) MarshalJSON() ([]byte, error) {
+	return json.Marshal(v.String())
+}
+
+func (v *Version) UnmarshalJSON(data []byte) error {
+	var version string
+	err := json.Unmarshal(data, &version)
+	if err != nil {
+		return err
+	}
+	*v = Parse(version)
+	return nil
+}