kcmodule: Avoid crashing in KAuth::ExecuteJob after closing window

Use a blocking job->exec() instead to access charge thresholds.

Async is nice, but will crash when pressing OK instead of Apply.
The window which has been set as parent window for the KAuth helper
dialog will disappear, and KAuth::ExecuteJob will crash inside
QWaylandIntegration::createPlatformWindow() which QWindow::winId()
depends on but fails. On Qt 6.6.1 at least.

Let's avoid this crash until there's a better fix.