feat: add n1n.ai API integration as model provider

[naaa760]

Pull Request Type

• [x] ✨ feat

Relevant Issues

resolves #4532

What is in this change?

Adds n1n.ai as a new LLM provider, providing access to 400+ large language models through a single API key.

Changes:

• Backend: Added N1nLLM provider with OpenAI-compatible API integration and agent support
• Frontend: Added n1n configuration component with API key input and model selection
• Integrated into onboarding flow and settings pages

Features:

• Supports 400+ models (OpenAI, Anthropic, Gemini, Llama, etc.)
• Streaming and non-streaming completions
• Agent workflow compatible

Additional Information

Configuration environment variables:

• N1N_API_KEY (required)

• N1N_MODEL_PREF (optional)

• Resources: https://n1n.ai/ | https://docs.n1n.ai/