Multi platform support

Right now, the Bespoke UI is a website from a local Python server. Users should have an easy way to learn on other platforms, at least:

- Android
- iOS

Learning should be possible offline after installation. Initially, assuming a single device without syncing between them is okay.

Optionally, we can add sync for the deck file with the user's learning progress. This would require that the user is online eventually. But it would also need more infrastructure for the online storage.