feat(chat): wire sign-in → chat, add /chat/ask, static fixes, env loading

[krishnavania09]

Summary

• Fixes navigation: home → sign-in → chat works end-to-end
• Adds /chat/ask JSON endpoint calling helper.send_gptnew()
• Uses url_for('static', ...) and removes broken image refs
• Loads .env (OPENAI_API_KEY) for LLM calls; safe fallback when model.pkl missing

Why

Previously users could reach only home/sign-in. This enables actual chat usage with minimal changes and keeps the original helper-based architecture.

Notes

• Text-only static (no images)
• No breaking changes to existing routes

[krishnavania09]

Ready for review @heathbrew

How to test

1. Create venv + install deps:
   python -m venv .venv
   . .venv/Scripts/activate
   pip install -r requirements.txt
2. Set key:
   $env:OPENAI_API_KEY="sk-..."
3. Run: python app.py
4. Sign in: student@example.com / student
5. Go to /chat → type a prompt → expect a JSON call to /chat/ask and a text answer.