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",

frontend/src/pages/admin/bills/BillDetailPage.tsx

@@ -526,6 +526,7 @@ export const BillDetailPage: React.FC = () => {
                   className="w-full px-3 py-2 rounded-md border border-neutral-300 dark:border-neutral-600 bg-white dark:bg-neutral-800 text-sm focus:outline-none focus:ring-2 focus:ring-primary-500 focus:border-accent-dark"
                 >
                   <option value="">{t('bills.payment.methodPlaceholder', 'Select method…')}</option>
+                  <option value="bank_transfer">{t('bills.payment.methods.bankTransfer', 'Bank transfer')}</option>
                   <option value="cash">{t('bills.payment.methods.cash', 'Cash')}</option>
                   <option value="card">{t('bills.payment.methods.card', 'Card')}</option>
                   <option value="paypal">{t('bills.payment.methods.paypal', 'PayPal')}</option>