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/postcss.config.mjs

@@ -0,0 +1,5 @@
+export default {
+  plugins: {
+    "@tailwindcss/postcss": {},
+  },
+};

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";
 

apps/web-next/src/styles/app.css

@@ -0,0 +1,10 @@
+@import "tailwindcss";
+@source "../../../../packages/core-ui/src";
+@source "../../../../packages/navigation/src";
+@source "../../../../packages/blog/src";
+@source "../../../../packages/marketing-pages/src";
+@source "../../../../packages/media/src";
+@source "../../../../packages/auth/src";
+@source "../";
+
+@import "../../../../packages/core-ui/src/styles/theme.css";