Recent changes to how average review time is calculated have caused several unit tests to fail:

• StatisticsHelperTests
• TimeSpanFormatterTests

Tasks:

• Investigate which tests are failing in the above test suites.
• Update the tests to match the new behavior for average review time calculation.
• Ensure all tests pass and are correctly aligned with the intended logic.
• Refactor or remove any obsolete assertions related to previous average review time behavior.