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>

tests/test_cli.py

@@ -1375,3 +1375,16 @@ def test_package_updates_failure_raises_clierror(monkeypatch):
     FakeRun(monkeypatch, "Error: node unreachable\n", returncode=1)
     with pytest.raises(cli.CliError):
         cli.package_updates("mainframe")
+
+
+# --- #14 ----------------------------------------------------------------------------
+
+
+def test_version_argv_and_strips(fake):
+    fake.result = ("2.4.0\n", "", 0)
+    assert cli.version() == "2.4.0"
+    assert fake.argv == ["cv4pve-cli", "--version"]
+
+
+def test_tested_version_constant():
+    assert cli.TESTED_VERSION == "2.4.0"