Add download button and mp4 format

[tedsecretsource]

Currently to download an audio you have to tap and hold on the UI for the audio and then select Save to device. Not only is this unintuitive, it only works if you play the audio back, start to finish, first.

This is a ticket to add a button that allows you to download the audio immediately, without having to listen to it first.

Also, the file should be downloaded as mpeg, specifically, mp4. This repo shows how to do that in the browser using ffmpeg.wasm!