perf(project): 看板列头菜单改为首次展开时才渲染菜单项

每列菜单原先预先挂载约 17 个菜单项，18 列的项目约占面板一半的 watcher。改为记录已展开过的列，首次展开时再渲染。
移动端 4 倍降速实测，18 列项目打开时长任务约减半（1357 → 694ms）。

resources/assets/js/pages/manage/components/ProjectPanel.vue

@@ -167,10 +167,11 @@
                                 v-else-if="!isDepartmentReadonly"
                                 trigger="click"
                                 size="medium"
+                                @visible-change="onColumnMenuVisible(column.id, $event)"
                                 @command="dropColumn(column, $event)">
                                 <Icon type="ios-more" />
                                 <EDropdownMenu slot="dropdown" class="project-panel-more-dropdown-menu">
-                                    <li class="project-panel-more-dropdown-warp">
+                                    <li v-if="columnMenuIds.includes(column.id)" class="project-panel-more-dropdown-warp">
                                         <ul>
                                             <EDropdownItem command="title">
                                                 <div class="item">
@@ -742,6 +743,8 @@ export default {
         return {
             loading: false,
 
+            columnMenuIds: [],
+
             nowTime: $A.dayjs().unix(),
             nowInterval: null,
             tableToday: $A.daytz().format('YYYY-MM-DD'),
@@ -1514,6 +1517,12 @@ export default {
             });
         },
 
+        onColumnMenuVisible(columnId, visible) {
+            if (visible && !this.columnMenuIds.includes(columnId)) {
+                this.columnMenuIds.push(columnId)
+            }
+        },
+
         dropColumn(column, command) {
             if (this.isDepartmentReadonly) {
                 return;