fix: resolve 404 balances endpoint and currency formatting errors

- Add missing /api/v1/balances endpoint to backend
- Update frontend Account type to match backend AccountDetails model
- Add currency validation with EUR fallback in formatCurrency function
- Update AccountsOverview, TransactionsList, and Dashboard components
- Fix balance calculations to use balances array structure
- All pre-commit checks pass

frontend/src/components/Dashboard.tsx

@@ -34,7 +34,11 @@ export default function Dashboard() {
     { name: 'Analytics', icon: BarChart3, id: 'analytics' as TabType },
   ];
 
-  const totalBalance = accounts?.reduce((sum, account) => sum + (account.balance || 0), 0) || 0;
+  const totalBalance = accounts?.reduce((sum, account) => {
+    // Get the first available balance from the balances array
+    const primaryBalance = account.balances?.[0]?.amount || 0;
+    return sum + primaryBalance;
+  }, 0) || 0;
 
   return (
     <div className="flex h-screen bg-gray-100">