Introduces IHubUserAccessor and its implementation to centralize user ID retrieval from SignalR HubCallerContext, replacing duplicated logic in ChatsHub, FriendsHub, and PresenceHub. Moves extension and mapping files to a Common directory, adds UserToUserDtoMappingAction for online status mapping, and implements OnlineUserStore with tests for tracking online users. Updates dependency injection and test code to use the new abstractions.
Artemy