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/hooks/use-auth-client.ts

@@ -0,0 +1,24 @@
+import { useSession } from "next-auth/react";
+import { useMemo } from "react";
+import LlamaStackClient from "llama-stack-client";
+
+export function useAuthClient() {
+  const { data: session } = useSession();
+
+  const client = useMemo(() => {
+    const clientHostname =
+      typeof window !== "undefined" ? window.location.origin : "";
+
+    const options: any = {
+      baseURL: `${clientHostname}/api`,
+    };
+
+    if (session?.accessToken) {
+      options.apiKey = session.accessToken;
+    }
+
+    return new LlamaStackClient(options);
+  }, [session?.accessToken]);
+
+  return client;
+}