fix: write STYLEREF by outline level, not style name

Word resolves the quoted style name against the localized built-in
name, so 'STYLEREF "Heading 1"' shows a field error in any
non-English Word. The numeric argument means outline level in Word
and LibreOffice alike (probed), and the importer already reads it.

src/lib/export/docx.ts

@@ -670,8 +670,10 @@ function inlineToRuns(content: TiptapNode[] = [], force: TextProps = {}): Inline
     } else if (node.type === 'pageCount') {
       out.push(new TextRun({ children: [PageNumber.TOTAL_PAGES], ...runPropsFromMarks(node.marks) }));
     } else if (node.type === 'chapterField') {
-      // Word's running head: STYLEREF picks the heading of that level in force on the page.
-      out.push(new SimpleField(`STYLEREF "Heading ${Number(node.attrs?.level) || 1}" \\* MERGEFORMAT`, String(node.attrs?.text ?? '')));
+      // Word's running head: STYLEREF picks the heading of that level in force on the
+      // page. The numeric form means outline level; a style name would be looked up
+      // localized ("Heading 1" errors in a German Word, which wants "Überschrift 1").
+      out.push(new SimpleField(`STYLEREF ${Number(node.attrs?.level) || 1} \\* MERGEFORMAT`, String(node.attrs?.text ?? '')));
     } else if (node.type === 'dateTimeField') {
       out.push(dateTimeRun(node));
     } else if (node.type === 'sequenceField') {

tests/unit/chapter-field.test.ts

@@ -36,6 +36,10 @@ describe('chapter field', () => {
 
   it('round-trips through DOCX as a STYLEREF field', async () => {
     const bytes = await buildDocx(doc, margins, 'portrait', hf);
+    // The numeric form (outline level) — Word resolves a quoted style name against the
+    // localized name, so "Heading 1" errors in any non-English Word.
+    const ftr = strFromU8(unzipSync(bytes)['word/footer1.xml']);
+    expect(ftr).toContain('w:instr="STYLEREF 1 \\* MERGEFORMAT"');
     const back = importDocx(bytes);
     // Word caches the shown name in the field result, which the import drops — the
     // level is what the live field needs.