load illust with id; context menu

lib/network/download.dart

@@ -233,6 +233,7 @@ class DownloadManager {
   }
 
   void addDownloadingTask(Illust illust) {
+    if(illust.downloaded || illust.downloading) return;
     var task = DownloadingTask(illust, receiveBytesCallback: receiveBytes, onCompleted: (task) {
       saveInfo(illust, task.imagePaths);
       tasks.remove(task);

lib/network/network.dart

@@ -391,4 +391,13 @@ class Network {
       return Res.fromErrorRes(res);
     }
   }
+
+  Future<Res<Illust>> getIllustByID(String id) async {
+    var res = await apiGet("/v1/illust/detail?illust_id=$id");
+    if (res.success) {
+      return Res(Illust.fromJson(res.data["illust"]));
+    } else {
+      return Res.error(res.errorMessage);
+    }
+  }
 }