Better icon for unknown models

[humphd]

We currently use the OpenAI logo when we don't have a specific logo.

For example, I followed @tarasglek walkthrough today to set up an account on Grok, made an API key, and add them as another free provider using the API URL https://api.groq.com/openai/v1.

Here I'm using Meta's new llama3-70b (wow it's fast!), but the icon makes it look like an OpenAI model.

We should probably do two things:

• Add icons for models that start with names we know (e.g., llama* could use some llama icon)
• otherwise, we should probably fallback to something like https://react-icons.github.io/react-icons/search/#q=robot