UI: 预览环境显示构建时间 + 售价默认299

- 预览栏显示部署时间（如 04/11 10:05）
- 售价默认299
- vite构建时注入__BUILD_TIME__

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/vite.config.js

@@ -1,7 +1,12 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
 
+const buildTime = new Date().toLocaleString('zh-CN', { timeZone: 'Asia/Shanghai', month: '2-digit', day: '2-digit', hour: '2-digit', minute: '2-digit' })
+
 export default defineConfig({
+  define: {
+    __BUILD_TIME__: JSON.stringify(buildTime),
+  },
   plugins: [vue()],
   server: {
     proxy: {