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

experimental/libbox/setup.go

@@ -10,6 +10,10 @@ func SetBasePath(path string) {
 	C.SetBasePath(path)
 }
 
+func SetTempPath(path string) {
+	C.SetTempPath(path)
+}
+
 func Version() string {
 	return C.Version
 }