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/messaege/CodeMessage.vue

@@ -7,10 +7,14 @@
       'full-screen': fullscreen,
     }"
   >
+    <legacy-el-icon
+      v-if="fullscreen"
+      name="el-icon-close"
+      @click="fullscreen = !fullscreen"
+    />
     <i
-      :class="
-        fullscreen ? 'el-icon-close' : 'iconfont icon-tubiao_chakangongyi'
-      "
+      v-else
+      class="iconfont icon-tubiao_chakangongyi"
       @click="fullscreen = !fullscreen"
     />
     <pre class="lum-scrollbar" v-html="formatCode(code, lang)" />