feat: 右下角添加回到顶部悬浮按钮，滚动超过300px显示

web/src/App.vue

@@ -54,14 +54,22 @@
         />
       </div>
     </div>
+
+    <!-- 回到顶部 -->
+    <transition name="fade">
+      <button v-if="showBackToTop" class="back-to-top" @click="scrollToTop" title="回到顶部">
+        <el-icon :size="20"><Top /></el-icon>
+      </button>
+    </transition>
   </div>
 
   <!-- 导入对话框（从订单页触发） -->
 </template>
 
 <script setup>
-import { ref, onMounted } from 'vue'
+import { ref, onMounted, onUnmounted } from 'vue'
 import { ElMessage } from 'element-plus'
+import { Top } from '@element-plus/icons-vue'
 import {
   setToken, getToken, removeToken,
   queryOrders, exportExcel, importLogistics, downloadTemplate, queryLogs, getStats
@@ -100,6 +108,15 @@ const logData = ref([])
 const logTotal = ref(0)
 const logLoading = ref(false)
 
+// 回到顶部
+const showBackToTop = ref(false)
+function handleScroll() {
+  showBackToTop.value = window.scrollY > 300
+}
+function scrollToTop() {
+  window.scrollTo({ top: 0, behavior: 'smooth' })
+}
+
 // 初始化
 onMounted(() => {
   const token = getToken()
@@ -112,6 +129,11 @@ onMounted(() => {
     handleQuery()
     loadStats()
   }
+  window.addEventListener('scroll', handleScroll)
+})
+
+onUnmounted(() => {
+  window.removeEventListener('scroll', handleScroll)
 })
 
 function handleLoginSuccess(data) {
@@ -282,4 +304,30 @@ function handleRefresh() {
   width: 100%;
   margin: 0 auto;
 }
+
+.back-to-top {
+  position: fixed;
+  right: 32px;
+  bottom: 32px;
+  width: 44px;
+  height: 44px;
+  border: none;
+  border-radius: 50%;
+  background: var(--primary-600, #2563eb);
+  color: #fff;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  box-shadow: 0 4px 12px rgba(37, 99, 235, 0.35);
+  transition: transform 0.2s, box-shadow 0.2s;
+  z-index: 1000;
+}
+.back-to-top:hover {
+  transform: translateY(-2px);
+  box-shadow: 0 6px 16px rgba(37, 99, 235, 0.45);
+}
+
+.fade-enter-active, .fade-leave-active { transition: opacity 0.25s; }
+.fade-enter-from, .fade-leave-to { opacity: 0; }
 </style>