Add comprehensive ViewModel unit tests

[MihaiCristianCondrea]

Summary

• make CodeViewModel accept an injected GetLessonUseCase so it can be supplied with fakes in tests
• add unit coverage for the Code, Help, Settings, and Support ViewModels using fake repositories and verifying delegation
• expand HomeViewModel tests to exercise promoted-app filtering and intent construction edge cases

Testing

• ./gradlew test (fails: Android SDK is not available in the execution environment)

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https://chatgpt.com/codex/tasks/task_e_68c8915ed4a8832da5019f5fd425794c