Port to Sandstorm

[graue]

Seems like a cool project — it's designed to make it super easy to run open source web apps on a personal server.

https://github.com/sandstorm-io/sandstorm/wiki/Porting-Guide

(It already supports another app that's described as "the ultimate music player", so this raises existential questions of should I even be making this silly project instead of using that one, if it's so good? 😀 But music players are as much about taste as functionality, and I'm far too stubborn to stop building Pots.)