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

🤖

go.mod

@@ -2,4 +2,4 @@ module git.smsvc.net/pomodoro/GoTomato
 
 go 1.23
 
-require github.com/gorilla/websocket v1.5.3 // indirect
+require github.com/gorilla/websocket v1.5.3