Custom OpenAI embedding endpoints

[mkagit]

As of lately Openrouter and others supports embedding through OpenAI compatible endpoint API, I suggest to update to support it for the community. Thanks

[sekikunn]

I tried to modify the OpenAI (API) part of the code in main.js to be compatible with the text-embedding-v4 from the Qwen3 embedding series, which is compatible with OpenAI, and added my API, but it failed. The notification in the upper right corner shows 0 token/sec.