Add AI assistant

[TommySailami]

Fixes issue #132

• Changed llm_client.py to use actual llm instead of mock responses.
• Utilized OpenRouter for model flexibility.
• Rearranged some code for better readability

May have introduced UI bug with AI assistant button in the nav bar going to a white screen.

[TommySailami]

Note: Openrouter api key needed for the ai assistant to work.

Please add it to your local .env file following this formatting:
OPENROUTER_API_KEY = YOUR_KEY_HERE