AI: Add in-game galaxy map as MapEditor canvas background

ZeroSpace/ZeroSpace.MapEditor/Views/MapEditorPage.xaml.cs

@@ -36,6 +36,21 @@ public partial class MapEditorPage : ContentPage
         MapView.Drawable = _drawable;
         MapView.SizeChanged += OnMapViewSizeChanged;
         TryAutoLoad();
+        LoadGalaxyMapAsync();
+    }
+
+    // ----- galaxy map background -----
+
+    private async void LoadGalaxyMapAsync()
+    {
+        _drawable.BackgroundImage = await MapBackground.LoadAsync();
+        MapView.Invalidate();
+    }
+
+    private void OnBackgroundToggled(object? sender, CheckedChangedEventArgs e)
+    {
+        _drawable.ShowBackground = e.Value;
+        MapView.Invalidate();
     }
 
     // ----- view transform -----