feat: a cell's vertical alignment survives saving

Import read style:vertical-align but neither exporter wrote it, so a
mid-cell callout label jumped to the row's top on every save. odf-kit's
CellOptions and docx's TableCell both take it natively; DOCX import maps
w:vAlign in as well.

src/lib/export/docx.ts

@@ -927,6 +927,9 @@ function tableToDocx(node: TiptapNode, contentWidthCm: number, num: Numbering):
           ? { marginUnitType: WidthType.DXA, top: cmToTwip(ownPad[0]), right: cmToTwip(ownPad[1]), bottom: cmToTwip(ownPad[2]), left: cmToTwip(ownPad[3]) }
           : undefined,
         shading: fill ? { type: ShadingType.CLEAR, fill } : undefined,
+        // Content set against the middle/bottom of the box → w:vAlign.
+        verticalAlign: cell.attrs?.verticalAlign === 'middle' ? 'center'
+          : cell.attrs?.verticalAlign === 'bottom' ? 'bottom' : undefined,
         width: widthTwip ? { size: widthTwip, type: WidthType.DXA } : undefined,
         borders: {
           top: docxCellBorder(cell.attrs, 'borderTop'),

src/lib/export/odt.ts

@@ -2669,6 +2669,9 @@ function exportTable(node: TiptapNode, doc: OdtDocument, contentWidthCm: number,
             const c = normalizeColor(bg);
             if (c) opts.backgroundColor = c;
           }
+          // Content set against the middle/bottom of the box → style:vertical-align.
+          const va = cell.attrs?.verticalAlign;
+          if (va === 'middle' || va === 'bottom') opts.verticalAlign = va;
           // Per-side borders → fo:border-* (null = the table default border below).
           for (const side of BORDER_SIDES) {
             const b = parseBorderAttr(cell.attrs?.[side] as string | null);