Mainly using SQLite and having access to native file system for reading and writing files. We wanted to provide a full offline functionality. While it's possible to achieve that in browser as well it seemed quite complicated for now (we might consider it in the future).

It’s actually fairly straightforward using the OPFS API; I used it to build an upload queue a while ago, so the user can drag arbitrary files on the browser, they get copied into the OPFS as regular files, and then I can upload them at my own leisure, even after browser restarts. The SQLite WASM build even has support for it AFAIR.

Can recommend, it’s a fun challenge :)