Preview deletion does not clean up veth network interface

[fedacking]

Problem

When a preview is destroyed via cmd_destroy() in preview.sh, the virtual ethernet interface pair (ve-<slug>) is not removed. nixos-container destroy does not always clean up the veth, leaving orphaned interfaces on the host.

Impact:

• Subsequent attempts to create a preview with the same slug fail with EEXIST
• Orphaned veth interfaces accumulate on the system
• Same issue exists in the rebuild path of cmd_update()

Location: server-config/preview.sh lines 619-660 (cmd_destroy())

Proposed Fix

Add veth cleanup after nixos-container destroy in both cmd_destroy() and cmd_update():

ip link delete "ve-${slug}" 2>/dev/null || true

Note: A fix already exists on branch fix/make-update-rebuild-config (PR #145) but has not been merged to main.