Access MainTabBarController through an abstraction

[staskus]

Description

A refactoring to avoid accessing MainTabBarController directly or through static methods.

To support a sidebar on iPad, we will likely need to create a new entity that encompasses both tab bar and split view functionality. Defining a clear public interface allows us to ensure that no functionality would be missed.

Testing information

This PR doesn't change any functionality. Only defines a protocol and accesses MainTabBarController through a shared entity rather than through a static method. However, I tested navigation through deep links, home actions, and generally within the app.

Screenshots

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[dangermattic]

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