feat(editor): admin-only raw HTML/CSS/JS embed node

Adds an htmlEmbed block node that renders and executes raw HTML/CSS/JS in the
wiki origin (e.g. an analytics tracker) — the owner-chosen variant C. Because
this is stored-XSS by design, only workspace admins/owners may get such a node
persisted; everyone executes it when reading.

- Node (editor-ext): htmlEmbed atom/isolating block; source stored base64 in
  data-source for lossless HTML<->JSON round-trip. renderHTML emits only the
  encoded marker (never inlines raw markup), so generateHTML/export/search are
  not themselves injection vectors. Registered in BOTH client extensions and
  server tiptapExtensions. Markdown round-trip via an <!--html-embed:b64-->
  comment (turndown) + a marked rule.
- Client NodeView: injects source and re-creates <script> elements so they
  actually run; edit modal; renders in read-only/share too. Slash item is
  admin-gated (adminOnly filtered by the user's workspace role).
- SERVER ENFORCEMENT (the real control — UI gating alone is insufficient):
  stripHtmlEmbedNodes() removes htmlEmbed from any document persisted by a
  non-admin, applied at every write path that introduces content from an
  untrusted author: collab onStoreDocument, REST/MCP/AI updatePageContent,
  single-file import, zip/multi-file import, page duplication, and transclusion
  unsync. Page restore introduces no new content. Public share/readonly viewers
  render fetched (already-stripped) content and do NOT open a collab socket, so
  the only residual is a transient broadcast window to concurrent authenticated
  editors (documented).

Implements docs/arbitrary-html-embed-plan.md (variant C).

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

apps/client/src/features/editor/extensions/extensions.ts

@@ -41,6 +41,7 @@ import {
   Drawio,
   Excalidraw,
   Embed,
+  HtmlEmbed,
   TiptapPdf,
   PageBreak,
   SearchAndReplace,
@@ -87,6 +88,7 @@ import CodeBlockView from "@/features/editor/components/code-block/code-block-vi
 import DrawioView from "../components/drawio/drawio-view";
 import ExcalidrawView from "@/features/editor/components/excalidraw/excalidraw-view-lazy.tsx";
 import EmbedView from "@/features/editor/components/embed/embed-view.tsx";
+import HtmlEmbedView from "@/features/editor/components/html-embed/html-embed-view.tsx";
 import PdfView from "@/features/editor/components/pdf/pdf-view.tsx";
 import SubpagesView from "@/features/editor/components/subpages/subpages-view.tsx";
 import TransclusionView from "@/features/editor/components/transclusion/transclusion-view.tsx";
@@ -365,6 +367,13 @@ export const mainExtensions = [
   Embed.configure({
     view: EmbedView,
   }),
+  // Raw HTML/CSS/JS node (Variant C). The node is registered for ALL users so
+  // documents authored by admins render correctly for everyone; INSERTION is
+  // gated to admins in the slash menu, and the server strips the node from any
+  // non-admin write so a non-admin cannot persist it.
+  HtmlEmbed.configure({
+    view: HtmlEmbedView,
+  }),
   TiptapPdf.configure({
     view: PdfView,
   }),