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.