Feature/custom OpenAI config

[viknesh-ai]

Summary

Added support for configuring a custom OpenAI-compatible API provider. This allows users to input a custom base URL, model name, and optional API key — enabling integration with tools like Ollama or LM Studio.
Helps developers test without requiring authentication or relying on hosted OpenAI APIs.

Fixes #N/A

Type of Change

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] Documentation
• [ ] Other: ___________

Testing

Manual testing in local development environment using a custom provider input in the Credentials UI.
Settings are persisted using Zustand store.
Could not verify with full end-to-end behavior due to login/auth limitation in local setup. Reviewers should focus on data binding and store updates.

Checklist

• [x] Code follows project style guidelines
• [x] Self-reviewed my changes
• [x] Tests added/updated and passing
• [x] No new warnings introduced
• [x] I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

Screenshots/Videos

[vercel[bot]]

Someone is attempting to deploy a commit to the Sim Studio Team on Vercel.

A member of the Team first needs to authorize it.

[Lutherwaves]

@viknesh-ai do you plan on continuing this one?