fix: Rename postcss.config.js to .cjs (ES module compatibility)

- Astro 5.x uses ES modules
- postcss.config.js was CommonJS syntax
- Rename to .cjs to fix build error
- Fixes: module is not defined in ES module scope

postcss.config.cjs

@@ -0,0 +1,6 @@
+module.exports = {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};