fix(ui): raise muted text opacities for WCAG AA contrast (#35)

The text-color ladder was Material Design's dark-theme values
(87/60/38% white), but those are tuned for Material's #121212
base. Our --color-surface-base is oklch(0.10 0 0), noticeably
darker, so the same opacities land below WCAG AA: text-tertiary
at 38% white over our base is only ~3.6:1 (AA needs 4.5:1),
which is why the Streams table's muted labels read as nearly
invisible.

Raise the ladder while preserving the primary > secondary >
tertiary hierarchy:
  primary   0.87 -> 0.92
  secondary 0.60 -> 0.75  (~AA)
  tertiary  0.38 -> 0.55  (~5.4:1, comfortably AA)

Reported in issue #35.

Author: DatanoiseTV