Hello there.
Here is how I see your project by milestones:
- Login / Register page (since you did not say what type of verification is needed, there will be none)
- Chat [server-side] (receives message from authenticated client -> sends it to other clients)
- Chat [client-side] (sends messages to server and receives responses from server).
However, you did not specify some details:
1. You described it as a mobile chat. That means the layout must be adapted to a mobile device?
2. You did not say anything about rooms. Everybody who is logged in will receive the messages. Am I correct?
Upd1: One last question. The chat will be accessed through a browser? (asking, because you indicated Swift as a technology for this project)