Use chat_id from the consultations payload for direct navigation and keep
the chat lookup as a fallback for legacy rows without chat_id.
- validate chat id in ChatRepository and make it nullable
- guard repeated taps while opening chat
- handle nullable chat id in ProfileCubit and EmptyChatScreen
related to #1106
The chat icon on the consultant consultation card had an empty onTap handler.
Resolve the conversation with the client via the chat API and navigate to it.
related to #1106
