Add golden testing for initial screen.

[polina-c]

Failure:

ib/editor/editor.dart:6:8: Error: Dart library 'dart:js_interop' is not available on this platform.
import 'dart:js_interop';
       ^
Context: The unavailable library 'dart:js_interop' is imported through these packages:

    /Users/polinach/_/dart-pad/pkgs/dartpad_ui/test/main_test.dart => package:dartpad_ui => dart:js_interop
    /Users/polinach/_/dart-pad/pkgs/dartpad_ui/test/main_test.dart => package:dartpad_ui => package:web => dart:js_interop
    ...

Detailed import paths for (some of) the these imports:
...

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/google_ai.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart