Selectable visual skin for <cngx-tab-group>. The skin is a pure thematic concern - every value renders the same tablist / tab /
tabpanel structure, slots, ARIA, and keyboard behaviour, and only
redirects CSS via the [data-skin] host attribute. 'line'
(default) is the underline indicator; 'contained' fuses the active
tab with the panel surface; 'segmented' sits the tabs in a muted
track and lifts the active tab onto a raised surface; 'pill'
renders rounded solid fills; 'pill-outline' renders the same pill
geometry with a tinted, outlined active state instead of a solid
fill.
Sized to the tab use cases - not a 1:1 copy of the stepper's
seven skins. The Material twin ([cngxMatTabs]) ignores this setting.