fix(format): accept explicit assetCode in formatAmount, default USDC not XLM

frontend/lib/format.ts

@@ -1,12 +1,30 @@
-const xlm = new Intl.NumberFormat("en-US", {
+
+const _numberFmt = new Intl.NumberFormat("en-US", {
   minimumFractionDigits: 2,
   maximumFractionDigits: 7,
 });
 
+/**
+ * FIX #18: formatAmount accepts an explicit assetCode instead of always
+ * appending "XLM". Defaults to "USDC" to match the product description
+ * (stablecoin rent payments), but every call site can override.
+ *
+ * Migration path for existing callers:
+ *   formatXLM(amount)           -> formatAmount(amount)          // shows USDC
+ *   formatXLM(amount)           -> formatAmount(amount, "XLM")   // old behaviour
+ */
+export function formatAmount(amount: number, assetCode = "USDC"): string {
+  return ;
+}
+
+/**
+ * @deprecated Use formatAmount(amount, assetCode) instead.
+ * Kept for incremental migration of existing call sites.
+ */
 export function formatXLM(amount: number): string {
-  return `${xlm.format(amount)} XLM`;
+  return formatAmount(amount, "XLM");
 }
 
 export function shortAddress(addr: string, head = 6, tail = 4): string {
-  return `${addr.slice(0, head)}…${addr.slice(-tail)}`;
+  return ;
 }