webui: fix Tailwind v4 utility classes missing when built via cmake (ggml-org#23253)

(cherry picked from commit 232f466)

Author: zoitrok

tools/server/webui/src/app.css

@@ -1,4 +1,5 @@
 @import 'tailwindcss';
+@source ".";
 
 @import 'tw-animate-css';