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/components/footnote/footnote.module.css

@@ -0,0 +1,106 @@
+/* Superscript reference marker. The visible number comes from the numbering
+   plugin decoration which sets the --footnote-number CSS variable. */
+.reference {
+  cursor: pointer;
+  color: var(--mantine-color-blue-6);
+  font-weight: 500;
+  vertical-align: super;
+  font-size: 0.75em;
+  line-height: 0;
+  user-select: none;
+  white-space: nowrap;
+}
+
+.reference::after {
+  content: var(--footnote-number, "");
+}
+
+.reference:hover {
+  text-decoration: underline;
+}
+
+.reference.selected {
+  background-color: var(--mantine-color-blue-1);
+  border-radius: 2px;
+}
+
+/* Read-only popover shown on hover/click of a reference. */
+.popover {
+  position: absolute;
+  z-index: 1000;
+  max-width: 360px;
+  padding: var(--mantine-spacing-sm);
+  background: var(--mantine-color-body);
+  color: var(--mantine-color-default-color);
+  border: 1px solid var(--mantine-color-default-border);
+  border-radius: var(--mantine-radius-md);
+  box-shadow: var(--mantine-shadow-md);
+  font-size: var(--mantine-font-size-sm);
+  line-height: 1.4;
+}
+
+.popoverHeader {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--mantine-spacing-xs);
+  margin-bottom: 4px;
+}
+
+.popoverNumber {
+  font-weight: 600;
+  color: var(--mantine-color-dimmed);
+}
+
+.popoverBody {
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
+/* Bottom footnotes container. */
+.list {
+  margin-top: var(--mantine-spacing-lg);
+  padding-top: var(--mantine-spacing-md);
+  border-top: 1px solid var(--mantine-color-default-border);
+}
+
+.listHeading {
+  font-weight: 600;
+  font-size: var(--mantine-font-size-sm);
+  color: var(--mantine-color-dimmed);
+  margin-bottom: var(--mantine-spacing-xs);
+  text-transform: uppercase;
+  letter-spacing: 0.03em;
+}
+
+.definition {
+  display: flex;
+  align-items: flex-start;
+  gap: var(--mantine-spacing-xs);
+  padding: 2px 0;
+}
+
+.definitionMarker {
+  flex: 0 0 auto;
+  min-width: 1.5em;
+  font-variant-numeric: tabular-nums;
+  color: var(--mantine-color-dimmed);
+  user-select: none;
+}
+
+.definitionContent {
+  flex: 1 1 auto;
+  min-width: 0;
+}
+
+.backLink {
+  flex: 0 0 auto;
+  cursor: pointer;
+  color: var(--mantine-color-blue-6);
+  user-select: none;
+  font-size: 0.9em;
+}
+
+.backLink:hover {
+  text-decoration: underline;
+}