fix: 导入 style.css 使全局色板令牌与 EP 主色覆盖生效

style.css 此前从未被导入，导致 --brand-red 等变量与
--el-color-primary 覆盖全部失效(死代码)，引发按钮透明不可见等问题。

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

offer_frontend/src/main.js

@@ -3,6 +3,7 @@ import { createPinia } from 'pinia'
 import ElementPlus from 'element-plus'
 import 'element-plus/dist/index.css'
 import zhCn from 'element-plus/dist/locale/zh-cn.mjs'
+import './style.css'
 import App from './App.vue'
 import router from './router'