Flag guests that no backup job covers

The guest table marks with "!" each guest that Proxmox's not-backed-up
list returns. The frame's bottom border shows the count, e.g.
"mainframe · 2 without backup job". This uses Proxmox's own answer, so
it also counts jobs that select guests by pool.

If the coverage call fails, the guests are still listed and a warning
says the coverage is unknown.

Closes #9

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

README.md

@@ -35,6 +35,9 @@ The shell key needs key-based root SSH to the PVE node (LXC only).
 Switching context runs `cv4pve-cli config use`, which changes cv4pve-cli's global current
 context: other shells and scripts follow, and it stays after the app exits.
 
+A `!` in the first column marks a guest that no backup job covers (Proxmox's
+`not-backed-up` list); the frame's bottom border shows how many.
+
 Empty fields in the edit dialog are left unchanged. If a CPU/memory change cannot be
 hot-plugged, the app tells you it applies at the next reboot.