Polish DESIGN.md visual depth

[peterabcd]

Summary

• apply a darker DESIGN.md hero treatment across key pages
• update session archive copy to “날짜, 장소, 참여 멤버로 세션을 찾아보세요.”
• refine PageHeader sizing and page hierarchy without adding new features

Verification

• pnpm run typecheck
• pnpm run lint
• pnpm run test
• pnpm run build
• git diff --check

Note: DESIGN.md remains local and untracked.