Improve dark brand portrait accessibility

[ajschlosser]

Summary

• add a dark-theme-specific negative portrait composited on the dark card surface
• keep the mark decorative so it does not duplicate localized product identity
• cover the decorative image semantics with a frontend regression test

Validation

• npm run typecheck
• npm run test:unit -- --run tests/frontend/brand-mark.test.ts tests/frontend/design-tokens.test.ts
• npm run build