Refine desktop Blog and News typography

[fanyangCS]

Summary

• apply the approved smaller/lighter 18px/500 headline scale to Blog and News lists on desktop
• set Blog excerpts to 15px/1.5 and list metadata to 12px/500
• preserve the 26px desktop page headings, existing spacing/separators, and existing mobile typography

Verification

• npx prettier assets/css/main.scss --check
• bundle exec jekyll build
• purgecss -c purgecss.config.js
• Playwright at 1200px and 1440px: heading 26px; entry title 18px/500/24.3px; Blog excerpt 15px/22.5px; metadata 12px/500
• Playwright at 390px: existing title 17px/600 retained; no horizontal overflow
• verified locally on Blog pages 1-2 and News pages 1-3

AI-assisted implementation and verification.