Light/dark and contrast quality filters (litmus-1wbl)


Status	Done · `archived`
Type	Feature
Priority	normal
Parent	(litmus-m8gs)

Add toggle filters above theme grid: light/dark mode filter and ‘good contrast only’ toggle that hides themes with WCAG AA failures.

Summary of Changes

Added filter controls to theme listing:

VariantFilter enum with All/Dark/Light modes using relative_luminance check
Good contrast toggle using validate_theme_readability
FilterButton component with active/inactive styling
Filtered theme count indicator
Empty state when no themes match
CSS: .filter-bar and .filter-btn styles