feat(editor): page templates - live whole-page embed (MVP)

Embed another page's LIVE content into a host page (it updates when the source
changes, not a static copy). A page can be flagged a template for discovery in
the picker; any accessible page can be embedded.

Server:
- migrations: pages.is_template (+ partial index) and page_template_references
  (whole-page back-refs); db.d.ts/entity types hand-merged (db.d.ts is curated).
- POST /pages/toggle-template (CASL Edit) flips is_template; is_template is
  returned by findById + the sidebar tree select so the tree menu label
  reflects state. Search suggestions gain an onlyTemplates filter for the picker.
- POST /pages/template/lookup ({sourcePageIds[]}, <=50): returns each accessible
  source's {title, icon, slugId, content, sourceUpdatedAt} with comment marks
  stripped (same access path as transclusion: filterViewerAccessiblePageIds;
  inaccessible -> no_access, missing -> not_found; error path -> not_found, never
  raw content).
- reference sync (collectPageEmbedsFromPmJson + syncPageTemplateReferences) on
  the Yjs save hook; duplicatePage remaps pageEmbed.sourcePageId + inserts refs.
  Known MVP gap: REST content updates don't resync refs (lookup uses in-doc ids).

Client:
- pageEmbed node (editor-ext, registered in BOTH client + server schemas);
  read-only NodeView with a batching lookup; '/Embed page' slash + template
  picker (self-embed prevented); 'Make/Unset template' in the tree node menu.
- Cycle guard: an ancestry-chain context + depth cap (5) render a 'circular
  embed' placeholder instead of recursing.
- Public shares show a placeholder (no public lookup in MVP).

MVP excludes (follow-ups): public-share lookup, unsync->static copy, server-side
expansion for export/RAG, MCP schema mirror, point-in-time snapshots.

Implements docs/page-templates-plan.md (MVP, variant A).

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

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

@@ -60,6 +60,7 @@ import {
   Status,
   TransclusionSource,
   TransclusionReference,
+  PageEmbed,
   TableView,
 } from "@docmost/editor-ext";
 import {
@@ -91,6 +92,7 @@ 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";
 import TransclusionReferenceView from "@/features/editor/components/transclusion/transclusion-reference-view.tsx";
+import PageEmbedView from "@/features/editor/components/page-embed/page-embed-view.tsx";
 import { common, createLowlight } from "lowlight";
 import plaintext from "highlight.js/lib/languages/plaintext";
 import powershell from "highlight.js/lib/languages/powershell";
@@ -230,7 +232,7 @@ export const mainExtensions = [
   Typography,
   TrailingNode,
   GlobalDragHandle.configure({
-    customNodes: ["transclusionSource", "transclusionReference"],
+    customNodes: ["transclusionSource", "transclusionReference", "pageEmbed"],
   }),
   TextStyle,
   Color,
@@ -381,6 +383,9 @@ export const mainExtensions = [
   TransclusionReference.configure({
     view: TransclusionReferenceView,
   }),
+  PageEmbed.configure({
+    view: PageEmbedView,
+  }),
   MarkdownClipboard.configure({
     transformPastedText: true,
   }),
@@ -420,7 +425,8 @@ const TEMPLATE_EXCLUDED_SLASH_ITEMS = new Set([
   "Draw.io (diagrams.net)",
   "Excalidraw (Whiteboard)",
   "Audio",
-  "Synced block"
+  "Synced block",
+  "Embed page"
 ]);
 
 const TemplateSlashCommand = Command.configure({