Warn on an untested cv4pve-cli version at startup

At startup the app runs `cv4pve-cli --version`. If it differs from
cli.TESTED_VERSION (2.4.0), a warning says which version was found
and which one was tested. The app parses output formats that are not
a stable API: the config list text, prune-backups as a JSON string,
the CT ip-addresses JSON string and the exit codes with --wait. A
newer cv4pve-cli could break them in confusing ways.

If cv4pve-cli cannot be run at all, an error says so once, instead of
every later call failing without explanation. The README notes the
tested version.

Closes #14

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

README.md

@@ -8,7 +8,8 @@ disk size. The table shows CPU % and Mem % of running guests and the info screen
 in-use values (refresh with `g`).
 
 It shells out to `cv4pve-cli` and uses its current context (`cv4pve-cli config use <name>`);
-it stores no credentials itself. Tested with cv4pve-cli 2.4.0.
+it stores no credentials itself. Tested with cv4pve-cli 2.4.0; with another version the app
+warns at startup, because it parses output formats that are not a stable API.
 
 ## Run