feat(quick-2): initialize v2 project from sample_interface

- Copy React app structure from sample_interface/src/
- Copy package.json, vite.config.ts, postcss.config.mjs
- Copy guidelines/ design system

v2/postcss.config.mjs

@@ -0,0 +1,15 @@
+/**
+ * PostCSS Configuration
+ *
+ * Tailwind CSS v4 (via @tailwindcss/vite) automatically sets up all required
+ * PostCSS plugins — you do NOT need to include `tailwindcss` or `autoprefixer` here.
+ *
+ * This file only exists for adding additional PostCSS plugins, if needed.
+ * For example:
+ *
+ * import postcssNested from 'postcss-nested'
+ * export default { plugins: [postcssNested()] }
+ *
+ * Otherwise, you can leave this file empty.
+ */
+export default {}