fix(invoices): add bank transfer to the mark-paid method list

The mark-paid dialog offered Cash / Card / PayPal / TWINT but not bank
transfer — the default method for the QR-bill / IBAN invoices picpeak issues
(createInvoice even falls back to 'bank_transfer'). Added it as the first
option. Backend already accepts paymentMethod as a free string, so no API
change; i18n bills.payment.methods.bankTransfer (de "Überweisung").

frontend/src/i18n/locales/de.json

@@ -4801,6 +4801,7 @@
       "notes": "Notizen",
       "methodPlaceholder": "Methode wählen…",
       "methods": {
+        "bankTransfer": "Überweisung",
         "card": "Karte",
         "cash": "Bar",
         "paypal": "PayPal",

frontend/src/i18n/locales/en.json

@@ -4789,6 +4789,7 @@
       "notes": "Notes",
       "methodPlaceholder": "Select method…",
       "methods": {
+        "bankTransfer": "Bank transfer",
         "card": "Card",
         "cash": "Cash",
         "paypal": "PayPal",