refactor: simplify listing fee to fixed 1 USD, remove multi-currency logic

js/services/btcpay.js

@@ -29,14 +29,13 @@ async function ensureModalLoaded() {
 /**
  * Create a payment invoice for a listing
  * @param {string} listingId - The listing UUID
- * @param {string} [currency='EUR'] - Payment currency
  * @returns {Promise<Object>} { invoiceId, checkoutLink, status, expirationTime }
  */
-export async function createInvoice(listingId, currency = 'EUR') {
+export async function createInvoice(listingId) {
     const response = await fetch(`${POW_SERVER}/btcpay/invoice`, {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ listingId, currency })
+        body: JSON.stringify({ listingId })
     })
 
     if (!response.ok) {