refactoring DataSync & SyncCubit

[nabildroid]

flashcards/lib/cubits/sync_cubit.dart

Line 97 in 821b751

// todo i think its better to move those function into SyncData static function

the core functionality of SyncCubit should be to ensure data synchronization between the server and the local repo
but it should not take the responsibility for merging between the cached data and online data

the current implementation violates the single responsibility principle
DataSync since it already uses the Models (flashcard, progress, statistics ..) it also should know how to merge them