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/page/tree/components/space-tree-node-menu.tsx

@@ -12,6 +12,7 @@ import {
   IconLink,
   IconStar,
   IconStarFilled,
+  IconTemplate,
   IconTrash,
 } from "@tabler/icons-react";
 
@@ -30,6 +31,7 @@ import {
   useRemoveFavoriteMutation,
 } from "@/features/favorite/queries/favorite-query";
 
+import { useToggleTemplateMutation } from "@/features/page-embed/queries/page-embed-query";
 import { treeDataAtom } from "@/features/page/tree/atoms/tree-data-atom.ts";
 import { treeModel } from "@/features/page/tree/model/tree-model";
 import { useTreeMutation } from "@/features/page/tree/hooks/use-tree-mutation.ts";
@@ -63,6 +65,26 @@ export function NodeMenu({ node, canEdit }: NodeMenuProps) {
   const addFavorite = useAddFavoriteMutation();
   const removeFavorite = useRemoveFavoriteMutation();
   const isFavorited = favoriteIds.has(node.id);
+  const toggleTemplate = useToggleTemplateMutation();
+  const isTemplate = !!node.isTemplate;
+
+  const handleToggleTemplate = async () => {
+    const next = !isTemplate;
+    try {
+      await toggleTemplate.mutateAsync({ pageId: node.id, isTemplate: next });
+      // Reflect the new flag locally so the menu label updates immediately.
+      setData((prev) =>
+        treeModel.update(prev, node.id, { isTemplate: next } as any),
+      );
+      notifications.show({
+        message: next
+          ? t("Page marked as template")
+          : t("Page is no longer a template"),
+      });
+    } catch {
+      // mutation surfaces the error via notifications
+    }
+  };
 
   const handleCopyLink = () => {
     const pageUrl =
@@ -217,6 +239,17 @@ export function NodeMenu({ node, canEdit }: NodeMenuProps) {
                 {t("Copy to space")}
               </Menu.Item>
 
+              <Menu.Item
+                leftSection={<IconTemplate size={16} />}
+                onClick={(e) => {
+                  e.preventDefault();
+                  e.stopPropagation();
+                  handleToggleTemplate();
+                }}
+              >
+                {isTemplate ? t("Unset as template") : t("Make template")}
+              </Menu.Item>
+
               <Menu.Divider />
               <Menu.Item
                 c="red"