chore: move src/llama_stack/ui to src/llama_stack_ui (#4068)

# What does this PR do?
This better separates UI from backend code, which was a point of
confusion often for our beloved AI friends.


## Test Plan
CI

src/llama_stack_ui/components/providers/session-provider.tsx

@@ -0,0 +1,7 @@
+"use client";
+
+import { SessionProvider as NextAuthSessionProvider } from "next-auth/react";
+
+export function SessionProvider({ children }: { children: React.ReactNode }) {
+  return <NextAuthSessionProvider>{children}</NextAuthSessionProvider>;
+}