Improve mobile readability on one course page

[beihaili]

Context

The course reader is used by beginners on mobile. Small layout improvements can make dense lessons easier to scan without requiring a broad redesign.

Suggested scope

Pick one course page, reader section, or reusable course component. Keep the change narrow.

Acceptance criteria

• Improve spacing, wrapping, tap target size, or visual hierarchy on one mobile course surface.
• Avoid broad redesigns or unrelated style changes.
• Include before/after mobile screenshots in the PR.
• Keep desktop layout stable.

Verification

• npm run lint
• npm test
• A mobile-width screenshot, ideally around 390px wide.