Adds fiat currency balances to user balances

Extends user balance information to include fiat currency balances,
calculated based on entry line metadata and account types.

This allows for a more comprehensive view of user balances,
including both satoshi and fiat currency holdings.

Updates the castle index template and API to display fiat balances.

static/js/index.js

@@ -506,6 +506,14 @@ window.app = Vue.createApp({
     formatSats(amount) {
       return new Intl.NumberFormat().format(amount)
     },
+    formatFiat(amount, currency) {
+      return new Intl.NumberFormat('en-US', {
+        style: 'currency',
+        currency: currency,
+        minimumFractionDigits: 2,
+        maximumFractionDigits: 2
+      }).format(amount)
+    },
     formatDate(dateString) {
       return new Date(dateString).toLocaleDateString()
     },