flutter on click download image from firebase

Question

Here, when I add a new picture, it appears here and it can be opened to a full screen, but when I click I want to download the picture that I long pressed on to the phone storage

StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection('gallery').snapshots(),
  builder: (context, snapshot) {
    return !snapshot.hasData ? Center(
      child: CircularProgressIndicator(),
    ) : Container(
      child: GridView.builder(
          itemCount: snapshot.data!.docs.length,
          gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 3
          ),
          itemBuilder: (context, index) {
            return Container(
              margin: EdgeInsets.all(3),
              child: FullScreenWidget(
                backgroundColor: Colors.white,
                child: FadeInImage.assetNetwork(
                    fit: BoxFit.contain,
                    placeholder: 'assets/loading.gif',
                    imageErrorBuilder:
                        (context, error, stackTrace) {
                      return Image.asset(
                          'assets/error.gif',
                          fit: BoxFit.fitWidth);
                    },
                    image: snapshot.data!.docs[index].get('url')),
              ),
            );
          }),
    );
  },
);