Fix Vite CJS Node API deprecation warning

Add "type": "module" to frontend/package.json so Vite loads
vite.config.ts as ESM instead of CJS. Convert the three CJS config
files to ESM:
- tailwind.config.js: require/module.exports → import/export default
- theme/colors.js: require → import
- postcss.config.js → postcss.config.cjs (PostCSS config stays CJS)

frontend/postcss.config.cjs

@@ -0,0 +1,14 @@
+/* eslint-disable */
+module.exports = {
+  plugins: [
+    require('postcss-preset-env')({
+      autoprefixer: {
+        flexbox: 'no-2009',
+      },
+      stage: 3,
+    }),
+    require('postcss-import'),
+    require('tailwindcss'),
+    require('autoprefixer'),
+  ],
+};