Add WeChat QR footer link

[trsoliu]

Summary

• add the WeChat official account QR image to docs public assets
• render the QR entry in the GitHub Pages footer and README
• add a docs footer check to keep the QR asset, label, and styling hook in place
• record the docs/README QR entry in the changelog

Verification

• pnpm typecheck
• pnpm test
• pnpm check:ui
• pnpm registry:build
• pnpm build:miniprogram
• pnpm build:docs
• pnpm --filter vekui test
• Playwright desktop/mobile footer QR render checks