feat: switch from Mistral to OpenAI gpt-4o-mini model

- Replace @ai-sdk/mistral with @ai-sdk/openai dependency
- Update model from mistral-medium-latest to gpt-4o-mini
- Use OPENAI_API_KEY from environment for authentication
- Improve conversation generation quality with OpenAI model

🤖 Generated with [Claude Code](https://claude.ai/code)

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

package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@ai-sdk/mistral": "^2.0.15",
+    "@ai-sdk/openai": "^2.0.32",
     "@mendable/firecrawl-js": "^4.3.5",
     "@radix-ui/react-progress": "^1.1.7",
     "@radix-ui/react-slot": "^1.2.3",