feat(editor): footnotes (reference + definitions model)

Adds footnotes: a superscript marker in the text linked to an editable
definition in a Footnotes section at the end of the page, with auto-numbering
and a read-only hover popover. Chose the reference+definitions model (3 plain
nodes) over an inline atom with a sub-editor specifically for collaboration
safety.

editor-ext (packages/editor-ext/src/lib/footnote/):
- footnoteReference (inline atom, id), footnotesList (block, last child),
  footnoteDefinition (paragraph+, id). renderHTML emits sup[data-footnote-ref]
  / section[data-footnotes] / div[data-footnote-def]; parse-rule priority makes
  the empty reference win over the Superscript mark (else it is dropped on the
  server save).
- numbering: a decoration-only plugin (pure function of doc order) -> every
  client computes identical numbers, no document mutation, Yjs-safe.
- sync plugin: single-pass, always SYNC_META-tagged and skipping remote txns
  (terminates, no loop), idempotent; canonicalizes to one trailing footnotesList
  (merging duplicates), creates missing definitions, drops orphans, and
  coexists with TrailingNode. Disabled in read-only.
- commands setFootnote (one tx: reference + definition at the matching index +
  focus) / removeFootnote (cascade, one undo) / scrollTo*. slash /footnote.

client: superscript NodeView + floating-ui read-only popover; bottom-list and
definition NodeViews; registered in mainExtensions.

server: the three nodes registered in tiptapExtensions so collab/save/export
keep them. Round-trip regression spec guards the Superscript parse-priority.

markdown: turndown/marked round-trip to pandoc/GFM [^id] (+ a code-fence guard
so footnote-like lines inside code blocks are not extracted).

MCP mirror: schema + markdown-converter + commentsToFootnotes rewritten to real
footnote nodes + diff marker counting; NUL sentinels written as \u0000 escapes.

v2 follow-ups (per plan): definition reordering on reference move, id-collision
regeneration on paste, multiple references to one footnote.

Implements docs/footnotes-plan.md (variant B).

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

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

@@ -61,6 +61,9 @@ import {
   TransclusionSource,
   TransclusionReference,
   TableView,
+  FootnoteReference,
+  FootnotesList,
+  FootnoteDefinition,
 } from "@docmost/editor-ext";
 import {
   randomElement,
@@ -91,6 +94,9 @@ 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 FootnoteReferenceView from "@/features/editor/components/footnote/footnote-reference-view.tsx";
+import FootnotesListView from "@/features/editor/components/footnote/footnotes-list-view.tsx";
+import FootnoteDefinitionView from "@/features/editor/components/footnote/footnote-definition-view.tsx";
 import { common, createLowlight } from "lowlight";
 import plaintext from "highlight.js/lib/languages/plaintext";
 import powershell from "highlight.js/lib/languages/powershell";
@@ -381,6 +387,19 @@ export const mainExtensions = [
   TransclusionReference.configure({
     view: TransclusionReferenceView,
   }),
+  FootnoteReference.configure({
+    view: FootnoteReferenceView,
+    // Skip orphan-cleanup on remote/collaboration steps so collaborating
+    // clients never fight over footnote integrity (deterministic numbering
+    // decorations handle the rest).
+    isRemoteTransaction: (tr: any) => isChangeOrigin(tr),
+  }),
+  FootnotesList.configure({
+    view: FootnotesListView,
+  }),
+  FootnoteDefinition.configure({
+    view: FootnoteDefinitionView,
+  }),
   MarkdownClipboard.configure({
     transformPastedText: true,
   }),