The simplest way is to add a web app manifest with "display": "standalone", and then ask the user to install the website from the browser.
Client-side storage is part of the web platform. Any website can store data locally.
Neutralino offers you the ability to use native methods to save data on your computer's storage. You can also use this to create portable save files that the user can keep and transfer over to different computers.
For Electron, there is an integration library called Greenworks. For Neutralino, I haven’t found anything similar, so it might be inconvenient to add Steam-specific functionality down the road.
I’m just raising this limitation because it’s one of a few concerns that steered me away from Neutralino for my Steam port project.