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/public/locales/ru-RU/translation.json

@@ -391,6 +391,13 @@
   "Toggle block": "Сворачиваемый блок",
   "Callout": "Выноска",
   "Insert callout notice.": "Вставить выноску с сообщением.",
+  "Footnote": "Сноска",
+  "Insert a footnote reference.": "Вставить ссылку на сноску.",
+  "Footnotes": "Примечания",
+  "Footnote {{number}}": "Сноска {{number}}",
+  "Go to footnote": "Перейти к сноске",
+  "Back to reference": "Вернуться к ссылке",
+  "Empty footnote": "Пустая сноска",
   "Math inline": "Строчная формула",
   "Insert inline math equation.": "Вставить математическое выражение в строку.",
   "Math block": "Блок формулы",