fix: add Tailwind CSS config to enable style compilation

Tailwind CSS 4 requires explicit content paths configuration. Without this,
the @tailwindcss/postcss plugin cannot find files to process, preventing
styles from being generated for the admin interface.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

admin/tailwind.config.ts

@@ -0,0 +1,11 @@
+import type { Config } from "tailwindcss";
+
+const config: Config = {
+  content: [
+    "./src/pages/**/*.{js,ts,jsx,tsx,mdx}",
+    "./src/components/**/*.{js,ts,jsx,tsx,mdx}",
+    "./src/app/**/*.{js,ts,jsx,tsx,mdx}",
+  ],
+};
+
+export default config;