ShimmerBlock
fun ShimmerBlock(height: Dp, modifier: Modifier = Modifier, widthFraction: Float = 1.0f, base: Color = MidnightColors.LightBarely, highlight: Color = MidnightColors.LightFaint)
A shimmering placeholder block for first-load skeletons — a luminance sweep across a rounded bar. Brand-neutral via base/highlight (default to the Midnight palette's faintest light tokens), so it reads as "loading" without introducing hue. Use one per line of content being awaited.