feat: Upgrade chat to Gemini Pro with configurable model

- Update Google AI model to gemini-pro-latest via env var
- Add GOOGLE_AI_MODEL environment variable for easy model switching
- Add initial greeting message explaining Ponderants features
- Re-add tool call handling to display node suggestions
- Fix chat authentication and streaming responses

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

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

.example.env

@@ -9,7 +9,8 @@ SURREALDB_PASS=root
 SURREALDB_JWT_SECRET=your-secret-key-here-change-in-production
 
 # Google AI API Key (for Gemini embeddings and chat)
-GOOGLE_AI_API_KEY=your-google-ai-api-key
+GOOGLE_GENERATIVE_AI_API_KEY=your-google-ai-api-key
+GOOGLE_AI_MODEL=gemini-pro-latest
 
 # Deepgram API Key (for voice-to-text)
 DEEPGRAM_API_KEY=your-deepgram-api-key