Add @testing-library/jest-dom dev dependency

Add missing test dependency required by setupTests.js to fix
test suite failure in GitHub Actions.

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

Author: tahierhussain

frontend/package-lock.json

@@ -70,6 +70,7 @@
   },
   "devDependencies": {
     "@babel/plugin-proposal-private-property-in-object": "^7.21.11",
+    "@testing-library/jest-dom": "^6.9.1",
     "eslint-config-google": "^0.14.0",
     "eslint-config-prettier": "^8.8.0",
     "eslint-plugin-prettier": "^4.2.1",