Add the task log: recent tasks with status, Enter shows the log

l on the main screen opens "Tasks", the recent cluster tasks newest
first. l on the info screen shows only that guest's tasks. Columns:
start, duration (or "running"), node, type, VMID ("-" for node
tasks), user and status. A failed task gets a "!" in the leading
column. Enter opens the task's log in a plain-text widget, so the
output is never parsed as markup.

`task list -o json` has no start or end times, so the list comes from
`/cluster/tasks`. The upid is validated (it must start with "UPID:")
before it goes in as a positional argument.

Review fixes:
- A finished task without a start time no longer crashes the app; its
  duration shows "-".
- "running" now means there is no endtime. A finished task with an
  empty status shows "unknown" and is flagged.

Read-only live check: 25 tasks listed, and the log of the newest
(vzdelsnapshot 104) ends with TASK OK.

Closes #10

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

README.md

@@ -28,6 +28,7 @@ uv run cv4pve-tui
 | `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>`) |
+| `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 |
 | `g` | refresh |
 | `c` | switch cv4pve-cli context (cluster) |
 | `q` | quit |