Implement Godot UX scene scaffold

src/ReactorMaintenance.Godot/Screens/GameOverScreen.cs

@@ -0,0 +1,19 @@
+using Godot;
+using ReactorMaintenance.Godot.Data;
+
+namespace ReactorMaintenance.Godot.Screens;
+
+public partial class GameOverScreen : ScreenBase
+{
+    public void Configure(AppController app, CampaignLevel level)
+    {
+        var body = CreatePage("Game Over");
+        body.Alignment = BoxContainer.AlignmentMode.Center;
+        body.AddChild(CreateBodyText($"Failed level: {level.Name}"));
+
+        var actions = new HBoxContainer { SizeFlagsHorizontal = SizeFlags.ShrinkCenter };
+        body.AddChild(actions);
+        actions.AddChild(CreateButton("Retry Current Level", app.RetryCurrentLevel));
+        actions.AddChild(CreateButton("Main Menu", app.ShowMainMenu));
+    }
+}