fix: add postcss.config.js so Tailwind CSS is actually compiled

Without this file Vite has no PostCSS plugins configured, so the
@tailwind base/components/utilities directives in main.css are silently
ignored and the built CSS file contains no styles at all.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}