I am very much interested in local-first/only (backend-less) PWAs, and have built a couple of them myself.
Unfortunately, I don't think I have any use for a time-boxing app. Nice work though.
Is there a reason you went with localStorage rather than indexedDB?
One useful thing is supporting 3rd party sync services. One of the apps I built allowed syncing data across devices via Dropbox. I never got Google Drive working, though, their API was horrible to work with in comparison. Is sync support something you've considered?