feat: add Tailwind CSS configuration and build setup

- Add package.json with Tailwind dependencies
- Add tailwind.config.js and postcss.config.js
- Add source tailwind.css file
- Generate tailwind.output.css for admin and vendor
- Add Tailwind CSS documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

postcss.config.js

@@ -0,0 +1,9 @@
+module.exports = {
+  plugins: [
+    require('tailwindcss'),
+    require('autoprefixer'),
+    require('cssnano')({
+      preset: 'default',
+    }),
+  ],
+}