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/src/services/ledgerService.js

@@ -390,7 +390,20 @@ function csvEscape(cell) {
   return `"${s.replace(/"/g, '""')}"`;
 }
 function minorToDecimal(m) { return ((Number(m) || 0) / 100).toFixed(2); }
-function dateOnly(d) { return String(d || '').slice(0, 10); }
+// yyyy-mm-dd, robust to Postgres returning DATE/TIMESTAMP columns as JS Date
+// objects (SQLite returns strings). String(dateObj).slice(0,10) yields
+// "Thu Jan 15", which Banana / accounting tools reject — so format the calendar
+// parts explicitly. Uses local parts (DATE columns come back at local midnight).
+function dateOnly(d) {
+  if (!d) return '';
+  if (d instanceof Date) {
+    const y = d.getFullYear();
+    const m = String(d.getMonth() + 1).padStart(2, '0');
+    const day = String(d.getDate()).padStart(2, '0');
+    return `${y}-${m}-${day}`;
+  }
+  return String(d).slice(0, 10);
+}
 
 const EXPORT_FORMATS = ['generic', 'banana', 'banana_ie', 'bexio'];
 

backend/src/services/taxReportService.js

@@ -978,6 +978,16 @@ async function renderTaxReportCsv({ from, to, currency, locale } = {}) {
 
   const minorToDotDecimal = (m) => ((Number(m) || 0) / 100).toFixed(2);
 
+  // yyyy-mm-dd, robust to Postgres returning dates as JS Date objects (SQLite
+  // returns strings) — raw String(dateObj) is "Thu Jan 15", not an ISO date.
+  const isoDate = (d) => {
+    if (!d) return '';
+    if (d instanceof Date) {
+      return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`;
+    }
+    return String(d).slice(0, 10);
+  };
+
   const typeLabelKey = (type) => (
     type === 'outgoing' ? 'tax_type_outgoing'
       : type === 'incoming' ? 'tax_type_incoming'
@@ -1013,7 +1023,7 @@ async function renderTaxReportCsv({ from, to, currency, locale } = {}) {
     lines.push([
       i + 1,
       t(useLocale, typeLabelKey(row.type)),
-      row.date,
+      isoDate(row.date),
       reference,
       row.party,
       row.eventName,