Frontend: Auto-append /api to URL

frontend/.env.example

@@ -1,2 +1,2 @@
-# Backend API URL (when deploying separately)
+# Backend API URL (without /api suffix - code adds it automatically)
 VITE_API_URL=https://nice-bill-agent-arena.hf.space

frontend/src/App.jsx

@@ -1,7 +1,11 @@
 import { useState, useEffect } from 'react'
 import { LineChart, Line, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer } from 'recharts'
 
-const API_BASE = import.meta.env.VITE_API_URL || '/api'
+const getApiBase = () => {
+  const url = import.meta.env.VITE_API_URL || ''
+  return url.endsWith('/api') ? url : url + '/api'
+}
+const API_BASE = getApiBase()
 
 function App() {
   const [runs, setRuns] = useState([])