add new API: GetAllFoldersAndInstallations

csharp/App/Backend/Controllers/Controller.cs

@@ -184,6 +184,25 @@ public class Controller
               .ToList(); // important!
     }
 
+    [Returns<TreeNode[]>]  // assuming swagger knows about arrays but not lists (JSON)
+    [Returns(HttpStatusCode.Unauthorized)]
+    [HttpGet($"{nameof(GetAllFoldersAndInstallations)}/")]
+    public Object GetAllFoldersAndInstallations()
+    {
+        var caller = GetCaller();
+        if (caller == null)
+            return new HttpResponseMessage(HttpStatusCode.Unauthorized);
+
+        using var db = Db.Connect();
+
+        var folders       = db.GetAllAccessibleFolders(caller) as IEnumerable<TreeNode>;
+        var installations = db.GetAllAccessibleInstallations(caller);
+        
+        return folders
+              .Concat(installations)
+              .ToList();  // important!
+    }
+    
     private static Folder PopulateChildren(Db db, Folder folder, HashSet<Int64>? hs = null)
     {
         // TODO: remove cycle detector