feat: add Node.js Express server for IAP and backend auth

Replace Python http.server with Express to handle:
- Large IAP JWT headers that caused 500 errors
- API request proxying with GCP identity tokens
- Static React build serving with SPA fallback

Update api.ts to use relative URLs in production for proxy routing.

src/lib/api.ts

@@ -1,6 +1,8 @@
 import type { ChatMode } from '@/types/chat'
 
-const API_BASE_URL = import.meta.env.VITE_API_URL || 'http://localhost:8000'
+// In production, use relative URL to route through the proxy server
+// For local development, set VITE_API_URL=http://localhost:8000 in .env
+const API_BASE_URL = import.meta.env.VITE_API_URL || '/api'
 const API_ENDPOINT = import.meta.env.VITE_API_ENDPOINT || '/chat/stream'
 const USE_MOCK_DATA = false // Set to true to use mock data for testing