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/.env.example

@@ -0,0 +1,11 @@
+# Frontend Environment Variables
+
+# API Base URL (optional)
+# - For local development: leave empty or don't set (will use /api)
+# - For production with separate backend: set to full URL
+# Example: https://api.yourdomain.com/api
+VITE_API_BASE_URL=
+
+# Note: When set, this URL is also used for WebSocket connections
+# http:// will be converted to ws://
+# https:// will be converted to wss://