feat(front-end): 初始化STP前端工程并提交页面框架

引入前端项目基础结构、页面骨架和UI组件，建立可运行的开发与构建配置，方便后续按模块迭代。

Co-authored-by: Cursor <cursoragent@cursor.com>

front-end/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 {}