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.

frontend/src/i18n/locales/de.json

@@ -3915,7 +3915,8 @@
       "intro": "Lade das Sammeljournal (Erträge + Kosten als periodengerechte Buchungssätze mit Konto- und MWST-Codes) zum Import in die Buchhaltungssoftware deines Treuhänders herunter.",
       "format": "Zielsoftware",
       "format_generic": "Generisch (alle Spalten)",
-      "format_banana": "Banana Buchhaltung",
+      "format_banana": "Banana — doppelte Buchhaltung",
+      "format_banana_ie": "Banana — Einnahmen/Ausgaben",
       "format_bexio": "bexio",
       "download": "CSV herunterladen",
       "failed": "Export fehlgeschlagen.",
@@ -3938,7 +3939,7 @@
     },
     "exportPdf": "PDF exportieren",
     "ledgerExport": "Treuhänder-Export",
-    "ledgerExportHint": "Doppelte Buchungssätze für Ihren Treuhänder, abgebildet über Ihren Kontenplan.",
+    "ledgerExportHint": "Buchungssätze für Ihren Treuhänder, abgebildet über Ihren Kontenplan.",
     "ledgerExportConfigure": "Einrichten →",
     "export": {
       "reportTitle": "Bericht",

frontend/src/i18n/locales/en.json

@@ -3915,7 +3915,8 @@
       "intro": "Download the collective journal (revenue + costs as accrual postings with account and VAT codes) for import into your Treuhänder’s accounting software.",
       "format": "Target tool",
       "format_generic": "Generic (all columns)",
-      "format_banana": "Banana Accounting",
+      "format_banana": "Banana — double-entry",
+      "format_banana_ie": "Banana — income & expense",
       "format_bexio": "bexio",
       "download": "Download CSV",
       "failed": "Export failed.",
@@ -3938,7 +3939,7 @@
     },
     "exportPdf": "Export PDF",
     "ledgerExport": "Accountant export",
-    "ledgerExportHint": "Double-entry postings for your accountant, mapped via your Chart of accounts.",
+    "ledgerExportHint": "Bookkeeping entries for your accountant, mapped via your Chart of accounts.",
     "ledgerExportConfigure": "Configure →",
     "export": {
       "reportTitle": "Report",