feat: allow restrict Pomodoro via password

- introduce password flag in server command line options
- update clientCommand model
- add password requirement for controlling the Pomodoro timer
- document password in README

internal/shared/state.go

@@ -16,3 +16,5 @@ func ResetToDefault() models.ServerMessage {
 		Paused:       false,
 	}
 }
+
+var PomodoroPassword string