feat(avatar): 默认头像轮前端集成——设置页两级样式 + 登录写学校默认键 + 顶栏三态头像

- BaseApi：schAvatarStyle/saveSchAvatarStyle/clearSchAvatarStyle 三端点（后端 SchSettingsController 配套，本仓 service 仓随轮提交）
- App.vue：登录后 loadSchAvatarDefault()（与 loadSchThemeDefault 同构：失败/404/空→不写键不报错不阻塞），写 localStorage[yh-avatar-sch-default]
- Settings.vue：新增 avatar pane（我的头像样式人人可见 + 本校默认管理员可配，watch 进 pane 拉取）
- AppHeader/UserInfoModal：头像展示位换 UserAvatar 三态组件
- ThemePreview：预览页头像形态联动
运行时联调与部署验证待人工（后端端点须随 service 发版）

src/components/AppHeader.vue

@@ -8,7 +8,7 @@
         <h4 class="fl align-middle">{{ appName }}</h4>
         <div class="school-bg"></div>
         <div class="header-aside fr">
-            <img :src="userInfo.profilePicture" @click="showUserInfo" class="fl align-middle" />
+            <UserAvatar :src="userInfo.profilePicture" :name="userInfo.name" :size="28" placement="brand" class="fl align-middle" @click="showUserInfo" />
             <a class="name fl align-middle text-ellipsis" @click="showUserInfo" href="javascript:void(0);">
                 Hi
                 <span class="ml-4">{{ userInfo.name }}</span>
@@ -27,6 +27,7 @@
 import { useDefaultStore } from "@/store";
 import { computed } from "vue";
 import SvgIcon from "@/components/SvgIcon.vue";
+import UserAvatar from "@/components/UserAvatar.vue";
 import mittBus from "@/utils/mitt";
 import { StateEnum } from "@/types";
 import { useRouter } from "vue-router";

src/components/UserInfoModal.vue

@@ -15,7 +15,7 @@
                 :headers="uploadHeaders"
             >
                 <div class="avatar-wrap" title="点击更换头像">
-                    <img :src="requireImg('user-avatar-default.png')" v-real-img="store.userInfo.profilePicture" alt="" />
+                    <UserAvatar :src="store.userInfo.profilePicture" :name="store.userInfo.name" :size="120" placement="light" />
                     <div class="avatar-mask">更换头像</div>
                 </div>
             </Upload>
@@ -67,8 +67,8 @@
 
 <script lang="ts" setup>
 import { ref, onMounted, onUnmounted, reactive } from "vue";
-import { requireImg } from "@/utils/hook/useImg";
 import { useDefaultStore } from "@/store";
+import UserAvatar from "@/components/UserAvatar.vue";
 import mittBus from "@/utils/mitt";
 import BaseApi from "@/api/BaseApi";
 import Utils from "@/utils";