Credential error persists: switch all 40 kubectl invocations from using
KUBECONFIG env var to the explicit --kubeconfig CLI flag. The flag has higher
precedence in kubectl's lookup order and is unambiguous regardless of any
inherited KUBECONFIG env var in the parent process environment.
Also adds test_kubectl_connection Tauri command (runs kubectl cluster-info
with the stored kubeconfig) and a Test button in Settings → Kubeconfig so
the exact kubectl output — context name, exit code, full stdout/stderr — is
visible without needing to inspect tracing logs. This output will reveal
whether the issue is expired certs, a missing exec-auth plugin, wrong context,
or something else entirely.
