Add context listing and switching to the TUI

Press c to list cv4pve-cli contexts and switch between them. The
active context is always shown in the header.

- Switching runs `cv4pve-cli config use`, which changes the global
  current context. cv4pve-cli 2.4.0 has no per-call context option.
- `config list` has no JSON output, so its text lines are parsed.
- The guest table is cleared as soon as another context is chosen, so
  the old cluster's guests cannot be acted on.
- A generation counter drops guest listings that started before a
  switch, and g and c are refused while a switch runs. Without this,
  a reload already in flight could refill the table with the old
  cluster's guests, and VMIDs collide across clusters.

103 tests pass. The switch was tested live from mainframe to
mainframe-edge and back.

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

README.md

@@ -21,8 +21,12 @@ uv run cv4pve-tui
 | `r` | reboot (confirm) |
 | `e` | edit cores / memory / grow a disk (disk growth asks again; it cannot be undone) |
 | `g` | refresh |
+| `c` | switch cv4pve-cli context (cluster) |
 | `q` | quit |
 
+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.
+
 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.