refactor(dashboard): dashboard.vue 改用 <script setup> 语法

代码评审发现 Options API 与项目 30+ 个页面（square.vue、
discover.vue、exhibition.vue 等）惯例不一致。改写为
<script setup>，并把 pageBg 提到模块顶层（不再通过 setup
return 传递）。Header 组件无 script 逻辑，无需改动。

frontend/pages/dashboard/dashboard.vue

@@ -23,14 +23,13 @@
   </view>
 </template>
 
-<script>
+<script setup>
 import { ref, onUnmounted } from 'vue'
 import DashboardHeader from './components/DashboardHeader.vue'
 import { useDashboardData } from '@/composables/useDashboardData'
 
-export default {
+const pageBg = 'linear-gradient(153deg, #FF9597 0%, #80DFFF 33%, #B8B8B8 74%, #D9D9D9 100%)'
-  components: { DashboardHeader },
+
-  setup() {
 const activeTab = ref('crystal')
 const starId = ref(uni.getStorageSync('star_id') || null)
 
@@ -45,18 +44,6 @@ export default {
 onUnmounted(() => {
   dispose()
 })
-
-    return {
-      activeTab,
-      loading,
-      error,
-      data,
-      isReady,
-      handleTabChange,
-      pageBg: 'linear-gradient(153deg, #FF9597 0%, #80DFFF 33%, #B8B8B8 74%, #D9D9D9 100%)',
-    }
-  },
-}
 </script>
 
 <style lang="scss" scoped>