feat(auth,ui): support github sign-in in the UI (#2545)

# What does this PR do?
Uses NextAuth to add github sign in support.

## Test Plan
Start server with auth configured as in
https://github.com/meta-llama/llama-stack/pull/2509


https://github.com/user-attachments/assets/61ff7442-f601-4b39-8686-5d0afb3b45ac

llama_stack/ui/lib/client.ts

@@ -1,6 +0,0 @@
-import LlamaStackClient from "llama-stack-client";
-
-export const client = new LlamaStackClient({
-  baseURL:
-    typeof window !== "undefined" ? `${window.location.origin}/api` : "/api",
-});