fix: register service worker with a relative path

[arnaudvalle]

If the app is run in a sub folder (like it is now with github pages), then it was pointing to a service worker in the root folder of the domain rather than the current folder of the app (i.e. /grejbox/)

Fixing error message when launching the app:

Uncaught (in promise) TypeError: ServiceWorker script at https://mcastagnetti.github.io/sw.js for scope https://mcastagnetti.github.io/ encountered an error during installation.

Test:

• Launch the app
• There should be no error message