feat: improve article reading navigation

[yitso]

Summary

• add article table of contents for h2/h3/h4 headings
• add stable heading anchors and restore direct hash deep-link scrolling
• add previous/next article navigation by publish date
• add responsive reading/navigation styles and zh/en labels

Validation

• reviewed against latest main; branch is not behind
• verified final PR scope is limited to 6 article-reading files
• full build passed on the final feature head: npm ci, pip install -r requirements.txt, npm run build:assets, and python -m app.build --static --skip-assets --site-config configs/example.json
• temporary validation workflow was run on a separate CI branch and is not part of this PR