simplified code view widget

[csells]

This PR removes the use of CodeMirror in the Generate New Code dialog as it's causing some display issues. It replaces that usage with a read-only TextField control.

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
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pkgs/samples/lib/hello_world.dart
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pkgs/samples/lib/sunflower.dart