fix(html-embed): address code-review findings on the sandbox commit

Follow-up fixes to the htmlEmbed-sandbox / trackerHead change:

- share-seo: inject trackerHead via a function replacer so `$`-sequences
  ($&, $', $`, $$) in the admin snippet are inserted literally instead of
  being treated as String.replace substitution patterns; warn when the
  </head> marker is absent instead of silently skipping injection.
- mcp: register a passthrough `htmlEmbed` node in the schema mirror so an
  AI/MCP edit of a page containing an embed no longer throws
  "Unknown node type: htmlEmbed" in TiptapTransformer.toYdoc.
- editor-ext + client: treat a non-finite `data-height` as auto (null) so a
  crafted/corrupted height cannot disable auto-resize or yield a NaN iframe
  height; extract a shared clampHeight helper.
- client: rename render-raw-html.{ts,test.ts} -> html-embed-sandbox.{...} and
  shouldExecute -> shouldRender so the seam name matches the sandbox model.
- client: i18n the iframe title; surface the real error reason in
  tracker-settings (console.error + err.response.data.message).
- docs: note hasHtmlEmbedNode is now a test-only helper; add an Unreleased
  CHANGELOG entry; drop the dangling "arbitrary HTML embed" planning-doc ref.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/editor-ext/src/lib/html-embed/html-embed.ts

@@ -97,7 +97,12 @@ export const HtmlEmbed = Node.create<HtmlEmbedOptions>({
         default: null,
         parseHTML: (el) => {
           const v = el.getAttribute("data-height");
-          return v ? parseInt(v, 10) : null;
+          if (!v) return null;
+          const n = parseInt(v, 10);
+          // A non-numeric data-height (e.g. crafted/corrupted import) must not
+          // become NaN: NaN is typeof "number" and would disable auto-resize and
+          // yield an unclamped iframe height downstream. Treat it as auto (null).
+          return Number.isFinite(n) ? n : null;
         },
         renderHTML: (attrs: HtmlEmbedAttributes) =>
           attrs.height ? { "data-height": String(attrs.height) } : {},