Refactor code to prepare for more code.

[polina-c]

This is large code moves to make it easier to review the UI change drafted here: #3235

1. Move large code elements out of main and widgets.
2. Rename widgets to simple_widgets.

No functional changes.

I will suggest more code reorg after merging the UI change.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// 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.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
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

[devoncarew]

a quick pass through -

[devoncarew]

Why this change?

[polina-c]

Because of isMac. Now it is utils.dart

[ditman]

If @devoncarew is happy, I'm happy!

[devoncarew]

lgtm!