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/eslint.config.mjs

@@ -1,22 +0,0 @@
-import { dirname } from "path";
-import { fileURLToPath } from "url";
-import { FlatCompat } from "@eslint/eslintrc";
-
-const __filename = fileURLToPath(import.meta.url);
-const __dirname = dirname(__filename);
-
-const compat = new FlatCompat({
-  baseDirectory: __dirname,
-});
-
-const eslintConfig = [
-  ...compat.extends("next/core-web-vitals", "next/typescript", "prettier"),
-  ...compat.plugins("prettier"),
-  {
-    rules: {
-      "prettier/prettier": "error",
-    },
-  },
-];
-
-export default eslintConfig;