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-lock.json

@@ -9,6 +9,7 @@
       "version": "0.1.0",
       "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",
@@ -67,6 +68,22 @@
         "zod": "^3.25.76 || ^4"
       }
     },
+    "node_modules/@ai-sdk/openai": {
+      "version": "2.0.32",
+      "resolved": "https://registry.npmjs.org/@ai-sdk/openai/-/openai-2.0.32.tgz",
+      "integrity": "sha512-p7giSkCs66Q1qYO/NPYI41CrSg65mcm8R2uAdF86+Y1D1/q4mUrWMyf5UTOJ0bx/z4jIPiNgGDCg2Kabi5zrKQ==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@ai-sdk/provider": "2.0.0",
+        "@ai-sdk/provider-utils": "3.0.9"
+      },
+      "engines": {
+        "node": ">=18"
+      },
+      "peerDependencies": {
+        "zod": "^3.25.76 || ^4"
+      }
+    },
     "node_modules/@ai-sdk/provider": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/@ai-sdk/provider/-/provider-2.0.0.tgz",