refactor: update daily challenge view test to use createTestWidget

[Copilot]

Updated the "should display app bar with correct title" test in daily_challenge_view_test.dart to use the existing createTestWidget() helper function instead of manually creating a MaterialApp instance.

Before:

await tester.pumpWidget(
  const MaterialApp(
    home: DailyChallengeView(),
  ),
);

After:

await tester.pumpWidget(createTestWidget());

This change:

• Makes the code consistent with other tests in the same file that already use createTestWidget()
• Eliminates code duplication by reusing the existing helper function
• Maintains exactly the same functionality since createTestWidget() returns an identical MaterialApp widget

Fixes #1655.

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