Show original brand icons for OpenRouter models

[quark-zju]

Right now, all models from OpenRouter use the OpenRouter icon. I'd like to change the branded model (ex. Gemini via OpenRouter) to the original brand's icon (ex. Google). Same for other brands.

Motivations of using OpenRouter instead of original service providers:

• There is no reliable way to enforce a spending limit for Google cloud billing. So I prefer Gemini via OpenRouter for financial safety reasons.
• Anthropic and OpenAI expire credit after about one year. Sometimes it feels difficult to use up the prepaid credits before they expire, OpenRouter might be a good alternative.

Right now, I still use OpenAI and Anthropic's official APIs and only use OpenRouter for Gemini. So I don't want to route everything via OpenRouter (yet).

I guess some people might still prefer the OpenRouter icon. So perhaps this might need to be some sort of configuration.

I can try to implement this feature if it looks reasonable. I'll appreciate some code pointers, though.