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_app.py

@@ -50,6 +50,7 @@ class Fake:
         self.use_error = None
         self.use_hook = None
         self.guests_by_context = {}
+        self.cli_version, self.version_error = "2.4.0", None
         self.unprotected, self.unprotected_error = set(), None
         self.list_hook = None
         # delivery A (guest info screen)
@@ -143,6 +144,11 @@ class Fake:
             raise self.unprotected_error
         return set(self.unprotected)
 
+    def version(self):
+        if self.version_error:
+            raise self.version_error
+        return self.cli_version
+
     def contexts(self):
         self.contexts_calls += 1
         if self.contexts_error:
@@ -307,6 +313,7 @@ def fake(monkeypatch):
         "bridges",
         "set_net",
         "not_backed_up",
+        "version",
         "snapshots",
         "take_snapshot",
         "rollback_snapshot",
@@ -3471,3 +3478,36 @@ async def test_frame_subtitle_singular_for_one_package_update(fake):
     async with app.run_test(size=SIZE) as pilot:
         await settle(app, pilot)
         assert frame_subtitle(app).endswith("· 1 package update")
+
+
+# --- #14: warn on an untested cv4pve-cli version -------------------------------------
+
+
+async def test_no_warning_for_the_tested_cli_version(fake):
+    app = make_app()
+    async with app.run_test(size=SIZE) as pilot:
+        await settle(app, pilot)
+        assert not any("tested with" in m for m, _ in app.seen)
+
+
+async def test_other_cli_version_warns_once(fake):
+    fake.cli_version = "2.5.0"
+    app = make_app()
+    async with app.run_test(size=SIZE) as pilot:
+        await settle(app, pilot)
+        warned = [m for m, sev in app.seen if sev == "warning" and "tested with" in m]
+        assert (
+            len(warned) == 1
+            and "2.5.0" in warned[0]
+            and cli.TESTED_VERSION in warned[0]
+        )
+
+
+async def test_cli_not_runnable_is_an_error_not_a_crash(fake):
+    fake.version_error = cli.CliError(
+        "[Errno 2] No such file or directory: 'cv4pve-cli'"
+    )
+    app = make_app()
+    async with app.run_test(size=SIZE) as pilot:
+        await settle(app, pilot)
+        assert any("cv4pve-cli" in m for m in errors(app)) and app.is_running