UI/Qt: Avoid repaint loop in vertical tab bar

Move tab button geometry updates out of TabBar::paintEvent(). That path
mutates child widgets, and Qt schedules more updates for those buttons.
During painting, that can keep the tab bar dirty and churn CPU.

Update buttons from layout, hover, and current-tab changes instead. Keep
hover bookkeeping in one helper, and skip widget mutators when they are
already in the desired state.