The installer and IPv6 probe found the external interface only through
`ip route show default` (the IPv4 main table). On IPv6-only hosts and
policy-routed boxes (v4 default in a non-main table, e.g. WARP/wireguard
tunnels) that returns nothing, so check-ipv6-support.sh died at
"no default route / ext interface found" before probing anything and
the installer silently fell back to a wrong iface.
Fall back to the IPv6 default route, then the first non-virtual UP
interface (excluding lo/incusbr/virbr/docker/veth/warp/wg), in all five
places that detect the WAN iface: check-ipv6-support.sh, 00-check.sh,
00-ip-ask.sh, uninstall.sh and DetectExtIF.
Verified on a real IPv6-only host: probe now reports ext iface eth0 and
reaches Phase 2 instead of dying.
Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
