[ML 0.5/MVI.02.2] Define and Implement UpdateMessageFlags UseCase (Flow-based)

Description

Implement the UpdateMessageFlags use case to handle changing message states (e.g., Flag.SEEN, Flag.DELETED).

Goal

Standardize flag updates using a reactive Flow that emits the result of local updates and remote synchronization status.

Acceptance Criteria

• The use case returns a Flow<Outcome<UpdateMessageFlags.Success, UpdateMessageFlags.Error>>.
• Updates the MessageStore via MessageRepository.
• Enqueues the appropriate flag synchronization command to the Backend, if needed.
• Testing: Unit tests using Fakes to verify that local state is updated and the correct backend sync call is initiated.

Technical Details

• The flow should emit as soon as the local update is successful and potentially again when the backend command is enqueued (if needed).