Upgrade model from gpt-4 to gpt-4o

[JiwaniZakir]

Problem

The model is hardcoded to gpt-4 in Chatbot.jsx. GPT-4o provides equivalent quality at lower cost and faster latency.

Proposed Solution

• Change the hardcoded model string to gpt-4o
• Optionally make it configurable via an environment variable: VITE_MODEL with a default of gpt-4o