chore: prepare for Vercel + Koyeb deployment

- API base URL now reads from VITE_API_URL env var (frontend)
- WorldMap localhost URLs replaced with env-aware API constant
- Added httpx to backend requirements (live data proxy)
- Added Procfile for Koyeb backend deployment
- Added vercel.json for frontend SPA routing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/vercel.json

@@ -0,0 +1,6 @@
+{
+  "buildCommand": "npm run build",
+  "outputDirectory": "dist",
+  "framework": "vite",
+  "rewrites": [{ "source": "/(.*)", "destination": "/index.html" }]
+}