[Feature Request]: support custom model#3

I would like to thank the author for open-sourcing AssisChat. After trying it, I feel it is not inferior to any paid iOS application. The AssisChat UI design is excellent and beautiful, and its sharing and keyboard extension capabilities are also very strong, solving many of my problems ;) Seeing that the project now supports OpenAI and Claude API, as well as custom API domain, is fantastic. If it could also add support for custom model names, it would allow more users of other services to use AssisChat, such as OpenRouter, Perplexity, and others. FYI, here is an example of openrouter API request: ```shell curl https://openrouter.ai/api/v1/chat/completions \ -H "Content-Type: application/json" \ -H "Authorization: Bearer $OPENROUTER_API_KEY" \ -d '{ "model": "openai/gpt-3.5-turbo", "messages": [ {"role": "user", "content": "What is the meaning of life?"} ] }' ```