It’s not possible to use EventSub or Webhooks without an internet accessible callback (and for EventSub it has to be over HTTPS too). Also for client-side apps you likely shouldn’t be using those services anyway as the user may not be able to open the appropriate ports, or use port forwarding, for your app.
For an app running on a users machine rather than your own server to detect follows you would simply poll the Get Users Follows endpoint https://dev.twitch.tv/docs/api/reference#get-users-follows once per minute, as that wouldn’t require a webserver to be running.