Setup page: offer lxd and snapcraft setup to remote visitors

Follow-up to #50 (closed).

Goal

The setup page (#50 (closed)) only offers tools whose setup can run on a machine that is not the pkgproxy host: apt, snap, go, docker, lxd-remote. pkgproxyctl setup lxd and setup snapcraft are left out because their first step is a curl --unix-socket <admin.sock> -X POST .../v1/lxd-provision against pkgproxy's local admin API (docs/design.md §16.9), which writes the apt/snapd config and https-rewrite hook that the LXD profile's disk devices bind-mount. They also detect the LXD bridge gateway on the machine they run on.

A remote LXD host that wants its containers to use a pkgproxy elsewhere on the LAN has no admin socket to call and no local files to bind-mount.

To discuss

• Can the profile carry the config itself (cloud-init, or environment.* plus file pushes) when pkgproxy is remote, instead of bind-mounting files the local daemon wrote?
• Should the host embedded in the container config be the pkgproxy LAN address, not the bridge gateway?
• Does the https-rewrite pre-start hook have a remote-friendly equivalent?