Improve authorize branding controls and preview

[markwylde]

Summary

• add a shared semantic branding package and normalize server-side branding defaults
• apply semantic branding variables across the user auth, portal, and authorize surfaces
• expand admin branding controls, dark/light authorize previews, and focused preview coverage
• document the branding improvement plan and current custom branding behavior

Checks

• pnpm tidy
• pnpm build
• pnpm --filter @DarkAuth/api --filter @darkauth/client --filter opaque-ts --filter @DarkAuth/user-ui test
• pnpm --filter @DarkAuth/branding test
• PW_REPORTER=dot PW_ARTIFACTS=off pnpm --filter @DarkAuth/test-suite exec playwright test tests/admin/branding/branding.spec.ts --reporter=dot --workers=1