Until now the desktop app only ran from a source checkout, so Pithos was
really usable only inside Theseus. electron-builder (the version Theseus
uses) now makes an NSIS installer and a portable exe; s3d is still fetched
on first run by the checksum-checked installer, so the app stays small and
never ships an s3d it did not verify. Icons are rendered from ui/icon.svg.
Quitting also no longer waits on idle keep-alive sockets before exiting.
