Add snapshots to the info screen: take, roll back, delete

A "Snapshots" section on the info screen lists the snapshots, newest
first, without Proxmox's "current" pseudo entry. Keys:

- p takes a snapshot. The name defaults to tui_YYYYMMDD_HHMM, the
  description is optional, and VMs can include RAM.
- R rolls back to the snapshot under the cursor. The confirm names the
  snapshot and its date. A running guest is started again; its status
  is read right before the rollback, and if it cannot be read the
  rollback does not run.
- D deletes the snapshot under the cursor, after a confirm.

`delete ct|vm <node> <vmid> -y` without the snapshot subcommand
destroys the whole guest. delete_snapshot therefore validates the name
first (letter first, then [A-Za-z0-9_-], not "current"), writes
"snapshot" into the argv as a literal, and checks the argv shape
before running it. A property test covers rejected names.

The info screen now scrolls, because four sections do not fit short
terminals. A description starting with "-" is refused, since it is a
positional argument and could be read as an option.

Live test on CT 104: take, roll back (started again) and delete. The
snapshot list, status, config and IPv4/IPv6 match the baseline.

Closes #8

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

README.md

@@ -23,6 +23,7 @@ uv run cv4pve-tui
 | `Enter` then `e` | in info: edit cores / memory / grow a disk (disk growth asks again; it cannot be undone) |
 | `Enter` then `b` | in info: backup now (snapshot, zstd, storage choice); `a` there = also show backups of earlier guests that had the same VMID |
 | `Enter` 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) |
+| `Enter` then `p` / `R` / `D` | in info: take a snapshot (name, description, VM: include RAM) / roll back to the snapshot under the cursor (confirms; starts the guest again if it is running) / delete the snapshot under the cursor (confirms) |
 | `t` | shell in a running LXC container (`ssh -t root@<node> pct enter <vmid>`) |
 | `g` | refresh |
 | `c` | switch cv4pve-cli context (cluster) |