feat(web-next): wire Tailwind CSS v4 via PostCSS

- Add @tailwindcss/postcss + postcss.config.mjs
- Create app.css with @source directives for monorepo packages
- Import app.css in root layout

apps/web-next/src/app/layout.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from "next";
+import "../styles/app.css";
 import { getNonce } from "@repo/core-shared/security/next";
 import { Providers } from "./providers";