🍒 SensorFaceController: use a separate QQmlEngine in createConfigUi

Config UI was using same engine as the applet, so it would inherit the colors wrongly from the Plasma Theme instead of the standard color scheme. This caused issues in mixed light/dark global themes.

Fix it by creating a new engine just for the config windows.

BUG: 483689 FIXED-IN: 6.0.5

(cherry picked from commit 3ffd61ba)