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>

apps/server/src/common/helpers/prosemirror/html-embed.util.ts

@@ -42,7 +42,9 @@ export function stripHtmlEmbedNodes<T = JSONContent>(pmJson: T): T {
 /**
  * Returns true if the document contains at least one `htmlEmbed` node anywhere
  * in its tree. Useful to decide whether a strip pass on the share read path
- * actually changed anything.
+ * actually changed anything. After the write-path role gate removal this is no
+ * longer called by production code; it is retained as a test-only assertion
+ * helper (and a detection primitive should a future read path need it).
  */
 export function hasHtmlEmbedNode(pmJson: unknown): boolean {
   if (!pmJson || typeof pmJson !== 'object') {