fix: parent-index wx:for 添加 wx:key 消除 WXML 运行时警告

- pendingTasks.slice(0,3) 提取为 topPendingTasks 计算属性，使
  uni-app 编译器能正确生成 wx:key="id"
- placeholder 文章占位卡片的 :key="'ph-'+i" 已改为 :key="i"

cfc-frontend/pages/index/parent-index.vue

@@ -423,6 +423,9 @@ export default {
     }
   },
   computed: {
+    topPendingTasks() {
+      return this.pendingTasks.slice(0, 3)
+    },
     energyScores() {
       var d = this.energyData
       if (!d) return { body: 0, mind: 0, wisdom: 0, action: 0, wealth: 0 }