App-wide cascade for the data-grid-accordion's default visual skin.
Resolution priority (high -> low):
Per-instance [skin] Input binding.
provideDataGridAccordionConfigAt(...) in a parent component's
viewProviders (component-scoped override).
provideDataGridAccordionConfig(...) at the application root.
Library default (unset - the base flat grid look).
The column template ([columns]) is deliberately absent: every grid carries a
different column set, so it stays a per-instance input with a base fallback and
has no app-wide default. Skin is the only surface with a plausible app-wide
default, mirroring withAccordionSkin.
App-wide default visual skin reflected onto [data-skin]. Unset by default
(the base flat grid look). A per-instance [skin] Input still wins; this
only moves the cascade default. Override via withDataGridSkin.