feat(render): full post-body pipeline + image proxy (#3976–#3978 intent)

[ety001]

Implements the complete post-body rendering pipeline on the next branch and absorbs the intent of master's image-proxy fixes #3976–#3978.

Those three commits target src/app/utils/ProxifyUrl.js, src/shared/HtmlReady.js, etc. — files that don't exist on next (it's a from-scratch rewrite). So this is a reimplementation of the same intent, not a cherry-pick: the current MarkdownViewer.tsx is a regex hack that doesn't even render ![]() images.

Pipeline (order is security-critical)

markdown-it (markdown→HTML) → HtmlReady (mutation) → sanitize-html (XSS)

sanitize-html is the real XSS filter (iframe whitelist, allowedTags/Attributes). Using bridge body_html directly would bypass it — unsafe. So we render raw markdown body and sanitize ourselves.

New files

• lib/media/proxify-url.ts — proxifyImageUrl. Absorbs all three fixes:
  • fix(images): proxy only first-party via /p #3976: only first-party image hosts go via /p/ (third-party passthrough)
  • fix(images): make /p/ URLs idempotent and normalize transform params #3977: idempotent /p/ URLs (no double base58-encode)
  • fix(images): canonicalize cover_image host before /p/ encoding #3978: steemitimages.com/D* → cdn.steemitimages.com inside the encoded URL
• lib/media/links.ts — URL/first-party/image regexes.
• lib/sanitize-config.ts — sanitize-html config: allowedTags/Attributes/Schemes, transformTags with the iframe whitelist verbatim (vimeo/youtube/3speak/soundcloud/twitch/dtube).
• lib/html-ready.ts — HtmlReady mutation: link normalize, iframe→videoWrapper wrap, image https-ify + clickable wrap, #tag/@mention linkify, naked-URL→img, proxifyImages. (MVP-scoped: video embed-replacement chains + external Phishing module not ported.)

Rewritten

• components/elements/MarkdownViewer.tsx — regex hack → real pipeline. Public props (text/className/large/hideImages) unchanged; PostFull unaffected.

Dependencies

markdown-it (actively-maintained CommonMark renderer, replaces unmaintained remarkable; identical options surface), bs58@6 (current ESM major), sanitize-html v2, @xmldom/xmldom, plus @types/{markdown-it,sanitize-html}.

Verification

• tsc --noEmit — clean
• eslint — clean
• next build — ✓ Compiled successfully
• Runtime-tested the proxy: first-party proxied via /p/:base58, idempotent under double-proxy, third-party passthrough, /D urls canonicalized to cdn inside the encoded URL

Not in scope

next/image (deliberately excluded — conflicts with /p/ double-optimization), voting-slider UX, master's non-image fixes.