Use StyleItem for item view background painting

Replace the custom Rectangle-based implementation in DefaultListItemBackground.qml with StyleItem.

This removes manual painting of alternating row backgrounds, hover states, focus indication and selection highlighting using Kirigami theme colors, and delegates rendering to the active QStyle instead.

Doing so improves consistency with widget-based item views and reduces duplication of style-specific rendering logic in QML by relying on the existing style implementation for item view painting.

systemsettings + Oxygen:

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