test: add react-devtools-demo extension artifacts from testing session

Generated during browser_install_popular_extension testing to verify
Chrome extension functionality works correctly.

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

react-devtools-demo/background.js

@@ -0,0 +1,14 @@
+// React DevTools Background Script (Demo)
+console.log('⚛️ React DevTools Demo Background Script loaded');
+
+// Listen for extension installation
+chrome.runtime.onInstalled.addListener(() => {
+  console.log('React DevTools Demo installed');
+});
+
+// Monitor for React pages
+chrome.tabs.onUpdated.addListener((tabId, changeInfo, tab) => {
+  if (changeInfo.status === 'complete' && tab.url) {
+    console.log('Page loaded, checking for React:', tab.url);
+  }
+});