feat(footnotes): multi-backlinks — definition returns to ALL its references (#168)

After #166 a repeated `[^a]` is one footnote (reuse): one number, one
definition, N forward links. But the definition's ↩ only returned to the
FIRST reference. Now a definition with N references shows ↩ a b c …, each
backlink scrolling to its own occurrence (Pandoc/Wikipedia convention); a
single-reference footnote keeps the plain ↩ unchanged.

- editor-ext: `computeFootnoteRefCounts(doc)` (id -> occurrence count) cached
  alongside the number map in the numbering plugin state; `getFootnoteRefCount`
  getter (O(1), no per-render doc walk). `scrollToReference(id, index?)` picks
  the index-th `sup[data-footnote-ref][data-id]` occurrence (document order),
  falling back to the first.
- client: FootnoteDefinitionView renders one lettered link (a, b, c, … aa …)
  per occurrence when refCount > 1; the chrome stays after the contentDOM so
  the #146 caret invariant holds. i18n keys (ru) added.

Tests: computeFootnoteRefCounts + getFootnoteRefCount (reuse counts, unknown
id => 0); structure test gains 3 cases (N lettered links render, click jumps
to the n-th occorrence, single ref => one ↩). NOTE: the visual layout of the
backlink row needs a real browser to verify (jsdom can't); the structural and
behavioral contract is covered headless.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/components/footnote/footnote.module.css

@@ -115,3 +115,18 @@
 .backLink:hover {
   text-decoration: underline;
 }
+
+/* Multi-backlink row (#168): ↩ a b c — one lettered link per reference
+   occurrence. Sits on the right, after the content, like the single ↩. */
+.backLinks {
+  flex: 0 0 auto;
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0.3em;
+  user-select: none;
+}
+
+.backLinkArrow {
+  color: var(--mantine-color-dimmed);
+  font-size: 0.9em;
+}