### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

The UI shows `{"error": "Missing onboarding session for invitation link."}` when attempting to complete the user onboarding flow if `litellm_key_header_name` is set to a custom header value.

The backend reads the onboarding JWT from the custom header configured via `litellm_key_header_name`, see https://github.com/BerriAI/litellm/blob/v1.92.0/litellm/proxy/proxy_server.py#L13351

The frontend normally propagates this header name after login, so authenticated API calls use the right header, but on the onboarding page the user has no session yet, so the frontend falls back to its default header (`Authorization`) and the claim request arrives with the token in `Authorization`, not `litellm_key_header_name`.

### Steps to Reproduce

1. Generate a fresh user onboarding link 2. With a different browser (or clear the browser cache), open the onboarding link 3. Set a password and click Sign Up

### Relevant log output

```shell

```

### What part of LiteLLM is this about?

UI Dashboard

### What LiteLLM version are you on ?

v1.92.0

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