Resolve offline behavior to avoid losing PWA support in upcoming Chrome

[grarer]

Chrome shows this warning in the browser console:

Site cannot be installed: Page does not work offline. Starting in Chrome 93, the installability criteria is changing, and this site will not be installable. See https://goo.gle/improved-pwa-offline-detection for more information.

Chrome 93 doesn't release until this fall, so it doesn't really affect this as a class project, but it's a good idea to resolve this if it wouldn't be too much effort.

[grarer]

if this were a real company, we would probably fix this bug over the summer to have everything ready in time for the new chrome release in the fall. since this project will be over by then, we don't need to fix this right now.