fix(docs): restore component page code highlighting

[trsoliu]

Summary

• render component Usage snippets through the shared TSX syntax highlighter
• make the components sidebar use one scroll container so the Components heading no longer masks nav items
• add docs regression checks for Usage highlighting and sidebar masking rules

Test Plan

• pnpm typecheck
• pnpm test
• pnpm check:ui
• pnpm registry:build
• pnpm build:miniprogram
• pnpm build:docs
• browser verification on /weapp/components/ for desktop and 390px mobile viewport