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/app/auth/signin/page.tsx

@@ -0,0 +1,118 @@
+"use client";
+
+import { signIn, signOut, useSession } from "next-auth/react";
+import { Button } from "@/components/ui/button";
+import {
+  Card,
+  CardContent,
+  CardDescription,
+  CardHeader,
+  CardTitle,
+} from "@/components/ui/card";
+import { Copy, Check, Home, Github } from "lucide-react";
+import { useState } from "react";
+import { useRouter } from "next/navigation";
+
+export default function SignInPage() {
+  const { data: session, status } = useSession();
+  const [copied, setCopied] = useState(false);
+  const router = useRouter();
+
+  const handleCopyToken = async () => {
+    if (session?.accessToken) {
+      await navigator.clipboard.writeText(session.accessToken);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    }
+  };
+
+  if (status === "loading") {
+    return (
+      <div className="flex items-center justify-center min-h-screen">
+        <div className="text-muted-foreground">Loading...</div>
+      </div>
+    );
+  }
+
+  return (
+    <div className="flex items-center justify-center min-h-screen">
+      <Card className="w-[400px]">
+        <CardHeader>
+          <CardTitle>Authentication</CardTitle>
+          <CardDescription>
+            {session
+              ? "You are successfully authenticated!"
+              : "Sign in with GitHub to use your access token as an API key"}
+          </CardDescription>
+        </CardHeader>
+        <CardContent className="space-y-4">
+          {!session ? (
+            <Button
+              onClick={() => {
+                console.log("Signing in with GitHub...");
+                signIn("github", { callbackUrl: "/auth/signin" }).catch(
+                  (error) => {
+                    console.error("Sign in error:", error);
+                  },
+                );
+              }}
+              className="w-full"
+              variant="default"
+            >
+              <Github className="mr-2 h-4 w-4" />
+              Sign in with GitHub
+            </Button>
+          ) : (
+            <div className="space-y-4">
+              <div className="text-sm text-muted-foreground">
+                Signed in as {session.user?.email}
+              </div>
+
+              {session.accessToken && (
+                <div className="space-y-2">
+                  <div className="text-sm font-medium">
+                    GitHub Access Token:
+                  </div>
+                  <div className="flex gap-2">
+                    <code className="flex-1 p-2 bg-muted rounded text-xs break-all">
+                      {session.accessToken}
+                    </code>
+                    <Button
+                      size="sm"
+                      variant="outline"
+                      onClick={handleCopyToken}
+                    >
+                      {copied ? (
+                        <Check className="h-4 w-4" />
+                      ) : (
+                        <Copy className="h-4 w-4" />
+                      )}
+                    </Button>
+                  </div>
+                  <div className="text-xs text-muted-foreground">
+                    This GitHub token will be used as your API key for
+                    authenticated Llama Stack requests.
+                  </div>
+                </div>
+              )}
+
+              <div className="flex gap-2">
+                <Button onClick={() => router.push("/")} className="flex-1">
+                  <Home className="mr-2 h-4 w-4" />
+                  Go to Dashboard
+                </Button>
+                <Button
+                  onClick={() => signOut()}
+                  variant="outline"
+                  className="flex-1"
+                >
+                  Sign out
+                </Button>
+              </div>
+            </div>
+          )}
+        </CardContent>
+      </Card>
+    </div>
+  );
+}