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>

src/cv4pve_tui/app.py

@@ -1040,6 +1040,7 @@ class GuestApp(App):
     def on_mount(self) -> None:
         # one place for every sub_title change: mount, switch and failed switch
         self.watch(self, "sub_title", self._show_context_frame)
+        self._check_version()
         table = self.query_one("#guests", DataTable)
         table.add_columns(
             "",  # "!" = no backup job covers the guest
@@ -1084,6 +1085,25 @@ class GuestApp(App):
         self.package_count = count
         self._update_frame()
 
+    @work(thread=True)
+    def _check_version(self) -> None:
+        try:
+            found = cli.version()
+        except Exception as exc:  # noqa: BLE001 - every later call fails too; say why once
+            self.call_from_thread(
+                self.notify,
+                f"cv4pve-cli is not runnable: {_message(exc)}",
+                severity="error",
+            )
+            return
+        if found != cli.TESTED_VERSION:
+            self.call_from_thread(
+                self.notify,
+                f"cv4pve-cli {found} found; this app was tested with {cli.TESTED_VERSION}. "
+                "Some screens may misread its output.",
+                severity="warning",
+            )
+
     def _show_context_frame(self, sub_title: str) -> None:
         self.context_name = sub_title.removeprefix("context: ")
         self._update_frame()

src/cv4pve_tui/cli.py

@@ -5,6 +5,10 @@ import json
 import re
 import subprocess
 
+# Output formats this app parses (config list text, prune-backups as a JSON string, the CT
+# ip-addresses JSON string, exit codes with --wait) were verified against this version.
+TESTED_VERSION = "2.4.0"
+
 
 class CliError(Exception):
     """cv4pve-cli failed; str() is its combined, stripped output."""
@@ -472,3 +476,7 @@ def task_log(upid: str) -> str:
     if not isinstance(upid, str) or not upid.startswith("UPID:"):
         raise ValueError("not a task id (UPID)")
     return run("task", "log", upid)
+
+
+def version() -> str:
+    return run("--version").strip()