frontend: KubeConfigLoader: Use Alert for error messages

The kubeconfig load dialog rendered errors in a hand-rolled Box with
a hardcoded red background and white text. The colors ignore the
active theme, and the message relies on color alone with no error
icon, unlike the rest of the app which uses the MUI Alert component
for this (e.g. ClusterGroupErrorMessage, NewProjectPopup).

Replace the Box with the standard Alert in error severity, and
regenerate the affected storyshots.