fix: resolve ReadableStream in simulator so LangGraph runs on RN

- Add src/polyfills.ts as the first executed module; inject ReadableStream/WritableStream/TransformStream via web-streams-polyfill and ponyfill fallback
- Import polyfills at the top of App.tsx so globals are set before any LangChain/LangGraph code loads
- Add assets/images/icon.png to fix Metro “Asset not found” for icon

App.tsx

@@ -4,6 +4,7 @@
  * Main application component with authentication routing.
  * Shows loading screen while restoring auth state.
  */
+import './src/polyfills';
 
 import React from 'react';
 import { Buffer } from 'buffer';