fix(accounting): emit ISO dates in exports (Postgres returns Date objects)

The Date column imported empty into Banana because dateOnly() did
String(d).slice(0,10) — on Postgres the date columns come back as JS Date
objects, so that yields "Thu Jan 15" instead of "2026-01-15", which Banana
rejects. (SQLite returns strings, so the tests never caught it — the
pg-date-serialisation trap.)

- ledgerService.dateOnly + taxReportService CSV now format Date objects to
  yyyy-mm-dd via local calendar parts (DATE columns are local-midnight).
- Regression test added with a real Date object (the existing tests all used
  string dates).

backend/__tests__/services/ledgerService.test.js

@@ -221,6 +221,20 @@ describe('exportPostings', () => {
     expect(contentType).toMatch(/text\/plain/);
   });
 
+  it('formats a Postgres Date object as yyyy-mm-dd (not "Thu Jan ...")', async () => {
+    // PG returns DATE columns as JS Date objects (SQLite returns strings); the
+    // export must still emit an ISO date, or Banana rejects it and the Date
+    // column imports empty.
+    invoiceRows = [{
+      id: 1, invoice_number: 'R-2026-0001', issue_date: new Date(2026, 0, 10),
+      vat_rate: 8.1, net_amount_minor: 10000, vat_amount_minor: 810, total_amount_minor: 10810,
+      customer_company_name: 'ACME',
+    }];
+    const { content } = await ledgerService.exportPostings({ ...period, format: 'banana' });
+    const dateCell = content.split('\r\n')[1].split('\t')[0];
+    expect(dateCell).toBe('2026-01-10');
+  });
+
   it('bexio format includes tax_code + currency', async () => {
     const { content } = await ledgerService.exportPostings({ ...period, format: 'bexio' });
     const header = content.split('\r\n')[0];