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/global/LegacyElIcon.vue

@@ -0,0 +1,25 @@
+<template>
+  <el-icon v-bind="$attrs">
+    <component :is="iconComponent" v-if="iconComponent" />
+  </el-icon>
+</template>
+
+<script>
+import { resolveLegacyIcon } from "@/core/legacy-icon-map";
+
+export default {
+  name: "LegacyElIcon",
+  inheritAttrs: false,
+  props: {
+    name: {
+      type: String,
+      default: "",
+    },
+  },
+  computed: {
+    iconComponent() {
+      return resolveLegacyIcon(this.name);
+    },
+  },
+};
+</script>