feat(server): restructure Pomodoro server into modular components

- move server logic to cmd/server/main.go
- create packages for websocket, pomodoro and broadcast handling
- define models for messages
- remove old GoTomato.go file
- update README

🤖

README.md

@@ -10,7 +10,7 @@ A pomodoro server written in Go
 
 ```
 docker run --rm -d --name pomodoro-client -v $PWD:/usr/share/nginx/html/ -p 8081:80 nginx
-go run .
+go run ./cmd/server
 ```
 
 open http://localhost:8081