feat: add social follow badges

[Vincentwei1021]

Summary

• add X, Douyin, and Red Note follow badges before the final Star History section in all three READMEs
• add two compact Gallery actions: Follow author expands to X, Douyin, and Red Note on hover/focus, followed by Star on GitHub
• use locally embedded brand SVGs for X, Douyin, Red Note, and GitHub
• keep the compact mobile category rail uncluttered by hiding the social block below 900px
• protect external Gallery links with noopener noreferrer

Accounts

• X: @VincentWei93
• Xiaohongshu: account 3653216544 (verified profile share link)
• Douyin: Vincent / account 335248116 (canonical public profile URL)

Verification

• python3 gallery/build-seo.py
• node --check gallery/translations.js
• node --check gallery/app.js
• git diff --check
• Chrome visual QA at desktop/dark theme and 390px mobile width
• verified all three Gallery links, accessible labels, and responsive overflow behavior