feat: Add environment variable support for API URL deployment

- Add VITE_API_BASE_URL environment variable support to api.ts
- Update WebSocket hook to use env variable for separate backend deployment
- Add frontend/.env.example with configuration documentation

This enables:
- Local development: uses /api (relative path)
- Production deployment: set VITE_API_BASE_URL to backend URL

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/hooks/useWebSocket.ts

@@ -47,8 +47,19 @@ export function useWebSocket(roomId: string | null, options?: UseWebSocketOption
       wsRef.current.close()
     }
 
-    const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:'
-    const wsUrl = `${protocol}//${window.location.host}/api/ws/${roomId}?token=${token}`
+    // Build WebSocket URL - use env variable or default to current host
+    const apiBaseUrl = import.meta.env.VITE_API_BASE_URL
+    let wsUrl: string
+
+    if (apiBaseUrl && apiBaseUrl.startsWith('http')) {
+      // Convert http(s) to ws(s)
+      const wsBase = apiBaseUrl.replace(/^http/, 'ws')
+      wsUrl = `${wsBase}/ws/${roomId}?token=${token}`
+    } else {
+      // Default: use current host
+      const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:'
+      wsUrl = `${protocol}//${window.location.host}/api/ws/${roomId}?token=${token}`
+    }
 
     setConnectionStatus('connecting')
     const ws = new WebSocket(wsUrl)

frontend/src/services/api.ts

@@ -1,6 +1,7 @@
 import axios, { type AxiosError, type InternalAxiosRequestConfig } from 'axios'
 
-const API_BASE_URL = '/api'
+// API Base URL: use environment variable or default to relative path
+const API_BASE_URL = import.meta.env.VITE_API_BASE_URL || '/api'
 
 // Create axios instance
 const api = axios.create({