webapp: No-network process-in-memory mode

[str4d]

We are currently using StreamSaver.js to stream the processed file back to the user, which requires using a service worker, and thus interacting with a server. It would be great to have a no-network mode that encrypts or decrypts entirely in memory (with corresponding memory limits) and then offers the resulting Blob to the user.

[str4d]

Another reason this would be useful is that the current StreamSaver.js approach requires a secure context, which you obviously don't get for a locally-hosted HTML file (but do get for a pre-cached webpage from a server that you then use while offline...)