Configure Tailwind v4 correctly for fumadocs v16

Co-authored-by: huangyiirene <7665279+huangyiirene@users.noreply.github.com>

Author: Copilot

apps/site/app/global.css

@@ -1 +1,5 @@
-@import "tailwindcss";
+@import 'tailwindcss';
+@import 'fumadocs-ui/css/neutral.css';
+@import 'fumadocs-ui/css/preset.css';
+
+@source '../node_modules/fumadocs-ui/dist/**/*.js';