Switch ChangeLogScreen from rootBundle to DefaultAssetBundle.of(context)
so the asset source can be overridden in widget tests. Add
test/widget/changelog_screen_test.dart with a happy-path test (asset loads
and content is rendered) and an error-path test (missing asset shows the
error message).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>