fix: a zone's own margins survive the ODF round trip

The Header/Footer style carries them, and the band that grows to hold them
is rebuilt from there on the way back in.

src/lib/export/odt.ts

@@ -3637,6 +3637,11 @@ function hfParaPropsXml(para: TiptapNode): string {
   if (align) props.push(`fo:text-align="${align}"`);
   const s = paraStyleFromAttrs(para.attrs);
   if (s.background) props.push(`fo:background-color="${s.background}"`);
+  // The zone's own margins, which grow the band on the way back in (import/CLAUDE.md).
+  for (const [attr, side] of [['spaceBefore', 'top'], ['spaceAfter', 'bottom']] as const) {
+    const v = para.attrs?.[attr];
+    if (typeof v === 'number' && v) props.push(`fo:margin-${side}="${v}pt"`);
+  }
   for (const [attr, side] of [
     ['borderTop', 'top'], ['borderRight', 'right'], ['borderBottom', 'bottom'], ['borderLeft', 'left'],
   ] as const) {

tests/unit/hf-zone-margins.test.ts

@@ -0,0 +1,34 @@
+// A header/footer zone's own vertical margins: LibreOffice keeps them on the Header /
+// Footer paragraph style, and a footer's space above is what grows the band.
+import { describe, it, expect } from 'vitest';
+import { unzipSync, strFromU8 } from 'fflate';
+import { buildOdt } from '../../src/lib/export/odt';
+import { importOdt } from '../../src/lib/import/odt';
+
+type N = any;
+
+const doc: N = { type: 'doc', content: [{ type: 'paragraph', content: [{ type: 'text', text: 'Body' }] }] };
+const zone = (attrs: N, text: string): N => ({
+  type: 'doc',
+  content: [{ type: 'paragraph', attrs, content: [{ type: 'text', text }] }],
+});
+const hf = {
+  header: zone({ spaceAfter: 34 }, 'Kopf'),
+  footer: zone({ spaceBefore: 24 }, 'Fuss'),
+  headerFirst: null, footerFirst: null, differentFirstPage: false,
+  headerEven: null, footerEven: null, differentOddEven: false,
+  pageCount: 1,
+};
+
+describe('header/footer zone margins', () => {
+  it('ride the ODF zone styles and come back', async () => {
+    const bytes = await buildOdt(doc, undefined, 'portrait', hf);
+    const styles = strFromU8(unzipSync(bytes)['styles.xml']);
+    expect(styles).toContain('fo:margin-bottom="34pt"');
+    expect(styles).toContain('fo:margin-top="24pt"');
+
+    const back = await importOdt(bytes);
+    expect(back.header?.content?.[0]?.attrs?.spaceAfter).toBeCloseTo(34, 1);
+    expect(back.footer?.content?.[0]?.attrs?.spaceBefore).toBeCloseTo(24, 1);
+  });
+});