[FEAT]: Add SuperGrok / xAI OAuth login (use SuperGrok subscription instead of API key)

What would you like to see?

Problem

Currently, AnythingLLM only supports the official xAI developer API (API key from console.x.ai). Many users (including myself) already pay for SuperGrok or SuperGrok Heavy. Tools like Pi already support logging in with the SuperGrok subscription via OAuth (device code / browser flow) and using the subscription quota instead of the paid-per-token API. It would be very valuable if AnythingLLM offered the same option.

Proposed solution

Add an alternative xAI authentication method:

• OAuth / SuperGrok login (similar to what Pi does with /login supergrok)
• Allow users to authenticate with their SuperGrok or SuperGrok Heavy account
• Route requests through the subscription-backed endpoints so usage counts against the SuperGrok quota instead of generating separate API bills

The existing API-key provider should remain available for users who prefer or need the official developer API.

Why this matters

• Many power users already pay for SuperGrok Heavy specifically for higher limits and better models.
• Forcing them to also pay per-token for the API creates double billing and friction.
• Other popular tools (Pi, some agent frameworks) already support this OAuth flow.
• It would make AnythingLLM much more attractive as a central multi-user frontend for people who want both local models and easy escalation to Grok Heavy.

Additional context

• xAI provides an OAuth 2.0 + PKCE / device-code flow that several open-source tools already use successfully.
• Ideal UX: a “Login with SuperGrok” button next to the normal API key field in the xAI provider settings.

Thank you for considering this!