Run fstrim in a VM via the guest agent

f on the info screen runs fstrim in a running VM with the guest agent
enabled. A container gets an info note (no guest agent; use pct
fstrim on the node). A stopped VM or one without an agent gets a
warning, and nothing runs. Otherwise a confirm comes first, then
agent ping and fstrim. The result reports the trimmed total
(GiB/MiB), the number of paths and any per-path errors.

Review fixes:
- Odd agent output no longer turns a finished trim into "fstrim
  failed". Unreadable output gives "no results". Odd entries are
  repaired: non-dict entries are dropped, a non-int trimmed becomes 0
  and a missing path becomes "?". Well-formed entries are returned
  unchanged.
- fstrim gets a 30-minute CLI timeout instead of 330 s, because a big
  disk can take longer.
- Notifications are never parsed as markup (GuestApp.notify defaults
  to markup=False). Guest names, paths and CLI or agent errors are
  untrusted text, so a "[/b]" in them could break the toast.
- f before the info screen has loaded says "still loading" instead of
  a wrong "not running".
- "1 path" in the singular.

Not run live: the issue asks for approval before fstrim on VM 108.
Only the harmless agent ping was run there, and it answered.

Closes #19

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

README.md

@@ -46,6 +46,7 @@ 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 `o` | in info: edit autostart, startup order (order, up and down delay) and tags (one confirmed `set config` call; empty fields remove the setting) |
 | `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) |
+| `Enter` then `f` | in info: fstrim in a running VM with the guest agent enabled (confirms; containers: use `pct fstrim` on the node) |
 | `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 |