Create separate cache repository layer:
- Add POfflineCacheRepository protocol for cache operations
- Add OfflineCacheRepository with CoreData and Kingfisher
- Add OfflineCacheSyncUseCase to coordinate sync workflow
- Update PBookmarksRepository to focus on API calls only
- Extend BookmarkEntityMapper with toDomain() conversion
UseCase coordinates between cache, API, and settings repositories
following dependency inversion principle.
