Add unit tests (#11) (#14)

## Summary
- Add Jest/RTL unit tests for data models, Card, Footer, CustomTextField/PasswordField, CustomToastMessage, ConversationCard, AuthContext, and SignIn
- Remove the broken CRA `App.test.tsx` placeholder that expected "learn react"
- Add `npm run test:ci` for non-interactive CI runs

Closes #11

## Test plan
- [ ] `cd llm-fe && npm install`
- [ ] `npm run test:ci` — expect 9 suites / 32 tests passing

Reviewed-on: #14

llm-fe/package.json

@@ -41,6 +41,7 @@
     "start": "NODE_ENV=development react-scripts start",
     "build": "NODE_ENV=production react-scripts build",
     "test": "NODE_ENV=development react-scripts test",
+    "test:ci": "CI=true NODE_ENV=development react-scripts test --watchAll=false --coverage=false",
     "eject": "react-scripts eject"
   },
   "eslintConfig": {