Fix avatar path resolution in dev-env

[rafaelbsky]

The avatar path is currently dependent on the CWD. So it might not work if executed from a different place, e.g., when using @atproto/dev-env as a dependency on another project.

A related but separate issue is that the img folder is not copied to dist/ on build.

This PR moves the images to an assets folder outside src and accesses it in a non-CWD-dependent way.

[matthieusieben]

I typically like to put "assets" in an assets folder (next to src). This way, if src gets discarded from the published package, the assets can still be bundled.

[devinivy]

Seems like a reasonable way to get this working in the short/med-term 👍