Fix backend import error and update proxy for local dev

- Add missing Optional import in backend/app/main.py
- Update Vite proxy to use localhost:8000 for local development
- Add package-lock.json from npm install

frontend/vite.config.js

@@ -8,7 +8,7 @@ export default defineConfig({
     port: 5173,
     proxy: {
       '/api': {
-        target: 'http://backend:8000',
+        target: 'http://localhost:8000',
         changeOrigin: true,
       }
     }