uploading files (not apps)

is there an easy way to view the sourcecode from an app (e.g. grocery) directly from the store?

Yep - there's a little GitHub cat logo just below each app's icon. Click on it and you go to the source :)

And is there a way to upload a regular file directly (I'm running linux if that matters and can launch a web app if needed). So I can just push the file towards my bangle.

There's no command-line way of doing it at the moment I'm afraid. You'd have to use the IDE.

It's possible that the Espruino CLI could be updated to allow Storage uploads, but that hasn't been done yet.