fix(accounting): Banana export is now a tab-separated .txt (actually importable)

Banana's "Text file with column headers" import (Actions → Import into
accounting) requires a TAB-separated .txt with unquoted values — picpeak was
emitting a comma-separated, quoted .csv, which won't even show in Banana's
*.txt file picker, let alone parse into columns.

- ledgerService.exportPostings: the `banana` format now serialises TAB-separated
  with no quoting, .txt extension, text/plain content-type. generic + bexio stay
  comma-CSV (RFC 4180). Tab/newline chars in a cell are collapsed to spaces.
- Frontend ledger.service: download filename uses .txt for banana.
- Tests updated for the new banana shape (tab header, .txt, text/plain).

The column names already matched Banana's NameXml; only the serialisation was
wrong. bexio left as comma-CSV (verify against bexio's import spec separately).

backend/__tests__/services/ledgerService.test.js

@@ -195,11 +195,16 @@ describe('exportPostings', () => {
     expect(filename).toMatch(/_generic\.csv$/);
   });
 
-  it('banana format uses Banana column names', async () => {
-    const { content, filename } = await ledgerService.exportPostings({ ...period, format: 'banana' });
+  it('banana format is a TAB-separated .txt with Banana column names', async () => {
+    const { content, filename, contentType } = await ledgerService.exportPostings({ ...period, format: 'banana' });
     const header = content.split('\r\n')[0];
-    expect(header).toBe('"Date","Doc","Description","AccountDebit","AccountCredit","Amount","VatCode"');
-    expect(filename).toMatch(/_banana\.csv$/);
+    // Banana's "Text file with column headers" import wants TAB-separated,
+    // unquoted values in a .txt — not a comma CSV.
+    expect(header).toBe('Date\tDoc\tDescription\tAccountDebit\tAccountCredit\tAmount\tVatCode');
+    expect(content.split('\r\n')[1]).toContain('\t');
+    expect(content).not.toContain('"');
+    expect(filename).toMatch(/_banana\.txt$/);
+    expect(contentType).toMatch(/text\/plain/);
   });
 
   it('bexio format includes tax_code + currency', async () => {

backend/src/services/ledgerService.js

@@ -407,8 +407,10 @@ async function exportPostings({ from, to, currency, format = 'generic' } = {}) {
   let headers; let rowOf;
 
   if (fmt === 'banana') {
-    // Banana "Conti doppia" import: Date, Doc, Description, AccountDebit,
+    // Banana "Conti doppia" import (Actions → Import into accounting → "Text
+    // file with column headers"): Date, Doc, Description, AccountDebit,
     // AccountCredit, Amount, VatCode. Amount = gross; VatCode expands VAT.
+    // Serialised TAB-separated + .txt below (Banana's required shape).
     headers = ['Date', 'Doc', 'Description', 'AccountDebit', 'AccountCredit', 'Amount', 'VatCode'];
     rowOf = (p) => [dateOnly(p.date), p.docNumber, p.description, p.debitAccount, p.creditAccount, minorToDecimal(p.grossMinor), p.vatCode];
   } else if (fmt === 'bexio') {
@@ -425,11 +427,23 @@ async function exportPostings({ from, to, currency, format = 'generic' } = {}) {
       p.vatCode, cur, minorToDecimal(p.grossMinor), minorToDecimal(p.netMinor), minorToDecimal(p.vatMinor)];
   }
 
-  const lines = [headers.map(csvEscape).join(',')];
-  for (const p of postings) lines.push(rowOf(p).map(csvEscape).join(','));
+  // Banana's "Text file with column headers" import (banana.ch doc node 9947)
+  // requires a TAB-separated .txt with UNQUOTED values — a comma .csv won't even
+  // appear in its *.txt file picker. generic / bexio stay comma-CSV (RFC 4180).
+  const isBanana = fmt === 'banana';
+  const sep = isBanana ? '\t' : ',';
+  // Tab layout: strip any tab/newline from a cell so it can't split the row;
+  // CSV cells go through the RFC-4180 quoter instead.
+  const fmtCell = isBanana
+    ? (v) => String(v == null ? '' : v).replace(/[\t\r\n]+/g, ' ')
+    : csvEscape;
+  const lines = [headers.map(fmtCell).join(sep)];
+  for (const p of postings) lines.push(rowOf(p).map(fmtCell).join(sep));
   const content = lines.join(eol) + eol;
-  const filename = `journal_${period.from}_to_${period.to}_${cur}_${fmt}.csv`;
-  return { content, filename, contentType: 'text/csv; charset=utf-8', count: postings.length };
+  const ext = isBanana ? 'txt' : 'csv';
+  const filename = `journal_${period.from}_to_${period.to}_${cur}_${fmt}.${ext}`;
+  const contentType = isBanana ? 'text/plain; charset=utf-8' : 'text/csv; charset=utf-8';
+  return { content, filename, contentType, count: postings.length };
 }
 
 // ── small util ───────────────────────────────────────────────────────