LLM-GENERATED TEXT

Follow-up to #680. In dark mode a view's primary color drives button fills, links, chart lines, and progress bars. One color can't pass contrast as both a primary button fill under white text (luminance 0.183 or less) and as small text on a dark card (0.256 or more), so any brand color fails one of the two.

To do

1. MDH-UI: a primaryTextColor prop on Layout that sets --mdhui-color-primary-text (and so -mark) separately from the fill. When it isn't given, the brand drives everything as it does now.
2. ViewBuilder: a picker for it in the view settings.
3. ViewBuilder: warnings in the UI when a chosen color misses the contrast floor for the places it's used: 4.5:1 for white text on a fill and for text on the card, 3:1 for chart marks.