feat(accounting): add a Banana "Income & Expense" (cash-book) export format

The Banana export assumed a double-entry file; a user importing into an Income
& Expense (Einnahmen-Ausgaben) file got "AccountDebit/AccountCredit/Amount/
VatCode column not found", since those columns only exist in double-entry.

Add a second Banana format alongside the double-entry one:
- ledgerService: new `banana_ie` format → Banana I&E columns Date, Doc,
  Description, Income, Expenses, ContraAccount (the income/expense account),
  VatCode (banana.ch doc 9946). Revenue → gross in Income + revenue account;
  cost → gross in Expenses + expense account. Same tab-separated .txt shape.
- Frontend: ExportFormat + dropdown gain `banana_ie`; .txt extension covers
  both Banana variants. Labels relabelled: "Banana — double-entry" and
  "Banana — income & expense" (de equivalents). Hint de-"double-entry"-fied.
- Test added for the I&E format.

Pairs with the prior UTF-8 BOM fix (the "·" mojibake). Tests + build green.

backend/__tests__/services/ledgerService.test.js

@@ -207,6 +207,20 @@ describe('exportPostings', () => {
     expect(contentType).toMatch(/text\/plain/);
   });
 
+  it('banana_ie format is Income & Expense columns, tab-separated .txt', async () => {
+    const { content, filename, contentType } = await ledgerService.exportPostings({ ...period, format: 'banana_ie' });
+    const [header, row] = content.trim().split('\r\n');
+    expect(header).toBe('Date\tDoc\tDescription\tIncome\tExpenses\tContraAccount\tVatCode');
+    // The mock period holds one revenue posting (gross 108.10) → Income filled,
+    // Expenses empty, ContraAccount = the revenue account.
+    const cells = row.split('\t');
+    expect(cells[3]).toBe('108.10'); // Income
+    expect(cells[4]).toBe('');       // Expenses
+    expect(cells[5]).not.toBe('');   // ContraAccount (revenue account)
+    expect(filename).toMatch(/_banana_ie\.txt$/);
+    expect(contentType).toMatch(/text\/plain/);
+  });
+
   it('bexio format includes tax_code + currency', async () => {
     const { content } = await ledgerService.exportPostings({ ...period, format: 'bexio' });
     const header = content.split('\r\n')[0];