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>

static/admin/css/tailwind.css

@@ -0,0 +1,4 @@
+/* Tailwind CSS source file for admin panel */
+@tailwind base;
+@tailwind components;
+@tailwind utilities;