* fix(panel): keep quota lane combo following external changes

An interactive pick severs the combo's declarative currentIndex binding,
so panelQuotaCombo stopped reflecting external cfg_panelQuotaLane changes
such as the restore-defaults action on the General page of the same
configuration dialog. Its sibling combos guard this with page-level
onCfg_*Changed sync handlers; give the quota lane combo the same handler
and a shared quotaLaneIndex helper, and pin the behavior in
tst_panel_style_settings.

* test(panel): simulate the pick by writing currentIndex before activated

ComboBox has no activate() invokable; reproduce the interactive pick the
way Qt performs it: an imperative currentIndex write that severs the
declarative binding, followed by the activated signal. Verified against
a plain-QtQuick ComboBox harness reproducing both the unfixed and fixed
behavior.