Add network editing with VLAN, firewall, IPv4/IPv6 and undo

Press n in the info screen to edit one interface: bridge, VLAN tag,
firewall, IPv4 (DHCP, static or none) and IPv6 (SLAAC, DHCP, static or
none). The netN string is parsed, the chosen fields are changed and
the string is written back, so hwaddr, model=MAC, mtu, rate, trunks
and type are kept. For VMs, IP settings go to cloud-init ipconfigN and
only when the VM has a cloud-init drive.

A confirm shows the old and new raw values before anything is sent.
Press u to restore the values from before the last change to that
guest in this session.

Review fixes:
- Undo entries are cleared on a context switch, because VMIDs repeat
  across clusters.
- has_cloudinit only looks at disks.
- CIDR needs a decimal prefix: no netmask form and no /0.
- Fields you did not touch are not rewritten.

Live test on CT 104: firewall, VLAN 99, IPv6 DHCP and static IPv4 were
each applied and undone, and net0 ended identical to the baseline.
Going from static back to DHCP left the CT without IPv4 until a
reboot, so the confirm now warns that IP changes on a running
container may need one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -21,11 +21,14 @@ uv run cv4pve-tui
 | `r` | reboot (confirm) |
 | `e` | edit cores / memory / grow a disk (disk growth asks again; it cannot be undone) |
 | `i` | info: backups, backup job, network (config and runtime IPs); `b` there = backup now (snapshot, zstd, storage choice); `a` there = also show backups of earlier guests that had the same VMID |
+| `i` then `n` | in info: edit network (bridge, VLAN, firewall, IPv4/IPv6; VM IP via cloud-init at next boot); `u` there = undo the last network change of this guest (this session only) |
 | `t` | shell in a running LXC container (`ssh -t root@<node> pct enter <vmid>`) |
 | `g` | refresh |
 | `c` | switch cv4pve-cli context (cluster) |
 | `q` | quit |
 
+Warning: link changes (bridge, VLAN, firewall) apply immediately on a running guest and can cut its network (and DNS for others if the guest is your DNS server). IP changes on a running container may need a reboot of the container (seen: static -> DHCP gave no IPv4 until a reboot).
+
 The shell key needs key-based root SSH to the PVE node (LXC only).
 
 Switching context runs `cv4pve-cli config use`, which changes cv4pve-cli's global current