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>

docs/plans/guest-tui-mvp.md

@@ -446,3 +446,69 @@ API facts (Proxmox VE 9.2, `api usage ... config set -v`):
       172.16.41.200/23 gw 172.16.40.1->DHCP. Bridge change cannot be tested live: the
       node has a single bridge (`vmbr0`). Final net0 string identical to the baseline.
 - [ ] Push `feat/network-edit`, merge to `main` (ask first).
+
+---
+
+# Snapshots in the info screen (issue #8)
+
+## Decision (user-approved)
+
+Info screen section "Snapshots"; keys `p` take, `R` roll back, `D` delete. LXC and VM.
+
+CLI facts (cv4pve-cli 2.4.0, read-only probes):
+
+- `get guest snapshots <node> <qemu|lxc> <vmid> -o json` -> list of dicts: `name`,
+  `description`, `snaptime` (epoch or null), `parent`, VM also `vmstate`. Always
+  contains a pseudo entry `name == "current"` ("You are here!") - not a snapshot.
+- `create ct|vm snapshot <node> <vmid> <snapname> <descr>` (+ `--vmstate 1` for VM RAM),
+  no `--yes`; with `--wait`.
+- `do rollback ct|vm <node> <vmid> <snapname>` needs `-y`; API param `start` (bool)
+  starts the guest after a successful rollback.
+- `delete ct|vm snapshot <node> <vmid> <snapname>` needs `-y`.
+  DANGER: `delete ct|vm <node> <vmid> -y` (without `snapshot`) DESTROYS THE WHOLE GUEST.
+
+### `cli.py`
+
+- `valid_snapname(name) -> str` - ValueError unless it matches
+  `[A-Za-z][A-Za-z0-9_-]{0,39}` and is not `current` (case-insensitive).
+- `snapshots(guest) -> list[dict]` - without `current`, newest first (`snaptime`
+  desc, null last).
+- `take_snapshot(guest, name, description="", vmstate=False)` ->
+  `create <kind> snapshot <node> <vmid> <name> <description or "cv4pve-tui">
+  [--vmstate 1 (VM only)] --wait`; name validated first.
+- `rollback_snapshot(guest, name, start=False)` ->
+  `do rollback <kind> <node> <vmid> <name> [--start 1] -y --wait`; name validated.
+- `delete_snapshot(guest, name)` ->
+  `delete <kind> snapshot <node> <vmid> <name> -y --wait`; name validated first, and
+  the argv is asserted to contain `snapshot` right after the kind and the name right
+  after the vmid (defence against ever sending the guest-destroying form).
+
+### `app.py` (UI contract)
+
+- InfoScreen gets `Vertical#snapshots_section` (border_title "Snapshots") with
+  `DataTable#snapshots` (Name, Date `YYYY-MM-DD HH:MM` or `-`, Description, and for VMs
+  RAM `yes`/`no`) and `Static#snap_note`. Loaded with the rest; a failure of
+  `cli.snapshots` is shown in `#snap_note` ("unavailable: <summary>"), the rest of the
+  screen still works. Empty list -> `#snap_note` "No snapshots".
+- `p`: `SnapshotScreen` dialog (DialogScreen style, title "Take snapshot: <name>
+  (<vmid>)"): `Input#snapname` prefilled `tui_YYYYMMDD_HHMM` (local time),
+  `Input#snapdesc`, `Checkbox#vmstate` (VMs only; not shown for CTs), buttons
+  `#yes`/`#no`, escape = no. Invalid name -> error notify, dialog stays, no call.
+  Yes -> busy guard, worker `cli.take_snapshot`, done/error notify, info reloads.
+- `R`: roll back the snapshot under the cursor of `#snapshots`. `ConfirmScreen`
+  titled "Roll back" naming snapshot and date and saying everything changed since then
+  is lost; if the guest is running (live status) it says it will be started again and
+  `start=True` is passed. Yes -> busy guard, worker, notify, info reloads.
+- `D`: delete the snapshot under the cursor. `ConfirmScreen` titled "Delete snapshot"
+  naming it. Yes -> busy guard, worker `cli.delete_snapshot`, notify, info reloads.
+- `R`/`D` with no snapshot -> info notify, no dialog.
+
+## Steps
+
+- [x] Tests - red first. - [x] Implementation - suite green, ruff clean, README keys.
+- Review fixes: rollback reads the live status right before rolling back (if it cannot
+  be read, no rollback); the info screen scrolls; a description starting with `-` is
+  refused (positional argument).
+- [x] Live on CT 104: take `tui_live_test`, listed; roll back (CT running -> started
+      again); delete; list back to the baseline (no snapshots); CT running.
+- [ ] PR with `Closes #8`, merge after user OK.