fix(im): 迁移 Element UI 图标并修复 Vue 3 兼容问题

- 新增 migrate-legacy-icons 脚本，批量将旧图标替换为 legacy-el-icon 组件
- 多个组件中将 <i class="el-icon-*"> 统一改为 <legacy-el-icon name="el-icon-*">
- 将 size="mini"/"medium" 调整为 Element Plus 的 small/default
- 新增 legacy-el-icon 全局组件及图标映射逻辑

im/src/components/chat/panel/PanelToolbar.vue

@@ -9,25 +9,25 @@
           class="multi-icon pointer"
           @click="$emit('event', 'merge_forward')"
         >
-          <i class="el-icon-position" />
+          <legacy-el-icon name="el-icon-position" />
         </div>
         <p>合并转发</p>
       </div>
       <div class="btn-group">
         <div class="multi-icon pointer" @click="$emit('event', 'forward')">
-          <i class="el-icon-position" />
+          <legacy-el-icon name="el-icon-position" />
         </div>
         <p>逐条转发</p>
       </div>
       <div class="btn-group">
         <div class="multi-icon pointer" @click="$emit('event', 'delete')">
-          <i class="el-icon-delete" />
+          <legacy-el-icon name="el-icon-delete" />
         </div>
         <p>批量删除</p>
       </div>
       <div class="btn-group">
         <div class="multi-icon pointer" @click="$emit('event', 'close')">
-          <i class="el-icon-close" />
+          <legacy-el-icon name="el-icon-close" />
         </div>
         <p>关闭</p>
       </div>