Show pending node package updates (read-only)

U opens "Package updates": the pending apt updates of every online
node, with node, package, current and new version, origin and
priority. A node that fails is shown as "<node>: unavailable", and the
other nodes are still listed. A hint says the list is as of the
node's last apt update; this call does not refresh it.

The main frame's bottom border shows "· N package updates" after the
backup count, with a singular for 1. The count is fetched on mount,
after a context switch and whenever the updates screen loads, not on
every guest reload, since it costs one CLI call per node. It uses the
same generation guard as the guest list, so a count from the old
cluster is dropped after a switch.

Running apt update or upgrade stays out of scope; it is a write.

Read-only live check: "mainframe · 23 package updates", and 23 rows
in the screen.

Closes #13

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

README.md

@@ -29,6 +29,7 @@ uv run cv4pve-tui
 | `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>`) |
 | `l` | tasks: recent cluster tasks (start, duration, node, type, VMID, user, status; `!` marks failures); `Enter` shows the log of the task under the cursor, `g` reloads. In info: `l` = only this guest's tasks |
+| `U` | package updates: pending updates per online node (node, package, current, new, origin, priority), read-only; `g` reloads. The list is as of the node's last `apt update`. The frame's bottom border shows the count |
 | `g` | refresh |
 | `c` | switch cv4pve-cli context (cluster) |
 | `q` | quit |