fix(ux): critique re-run findings — dark-class leak, anchor scroll margins, hero CDL typo, error contrast, setup dedup, skip link

[Bissbert]

Summary

• Removes leftover dark-mode utility classes (site has no dark mode) across confidence tap, export/import panel, learn quiz widget, rationale panel, and study settings panel
• Adds scroll-margin-top to anchor targets so in-page jumps clear the sticky header
• Fixes hero CDL notation typo on the homepage
• Bumps form/validation error text contrast to meet WCAG AA
• De-duplicates the quiz setup screen (removes the redundant "Quiz categories" list now that the category selector covers it)
• Adds a skip-to-content link for keyboard/screen-reader navigation

Test plan

• npm test (498 tests passing)
• npm run build

[github-actions]

🚀 Preview deployed to: https://63e1bba9.gemmology-dev.pages.dev