Refactor processing logic and enhance error handling

- Cleaned up code in processing.py by removing inline semicolons and improving readability.
- Updated upsert_file_relation calls to ensure consistent handling of file relations.
- Enhanced query_file_relations in db_schema.py to support filtering by file existence.
- Improved API error handling in index.ts with user-friendly messages for 401 and 403 errors.
- Added online/offline status tracking in Layout.vue.
- Implemented debounced search functionality across multiple views to optimize performance.
- Introduced loading skeletons in Dashboard.vue for better user experience during data fetching.
- Enhanced file preview cleanup logic in Images.vue, Orders.vue, and Tables.vue to prevent memory leaks.
- Updated global styles to include new loading and notification animations.

web/frontend/src/views/Layout.vue

@@ -86,6 +86,11 @@
           <h2 class="page-title">{{ pageTitle }}</h2>
         </div>
         <div class="topbar-right">
+          <!-- Online indicator -->
+          <div v-if="!isOnline" class="offline-badge">
+            <span class="offline-dot"></span>
+            离线
+          </div>
           <el-dropdown @command="handleCommand" trigger="click">
             <div class="user-chip">
               <div class="user-avatar">{{ (authStore.username || 'U')[0].toUpperCase() }}</div>
@@ -135,7 +140,7 @@
 </template>
 
 <script setup lang="ts">
-import { ref, computed, reactive } from 'vue'
+import { ref, computed, reactive, onMounted, onUnmounted } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 import {
@@ -152,6 +157,20 @@ const authStore = useAuthStore()
 const isCollapse = ref(false)
 const showPwd = ref(false)
 const pwdForm = reactive({ old_password: '', new_password: '' })
+const isOnline = ref(navigator.onLine !== false)
+
+// Track online/offline status
+function updateOnlineStatus() {
+  isOnline.value = navigator.onLine !== false
+}
+onMounted(() => {
+  window.addEventListener('online', updateOnlineStatus)
+  window.addEventListener('offline', updateOnlineStatus)
+})
+onUnmounted(() => {
+  window.removeEventListener('online', updateOnlineStatus)
+  window.removeEventListener('offline', updateOnlineStatus)
+})
 
 const navItems: { path: string; label: string; icon: any; badge?: string }[] = [
   { path: '/', label: '处理中心', icon: HomeFilled },
@@ -415,6 +434,27 @@ async function changePassword() {
   color: var(--text-primary);
 }
 
+/* ── Offline badge ── */
+.offline-badge {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 4px 12px;
+  border-radius: var(--radius-sm);
+  background: rgba(239,68,68,0.1);
+  color: #ef4444;
+  font-size: 12px;
+  font-weight: 600;
+}
+
+.offline-dot {
+  width: 6px;
+  height: 6px;
+  border-radius: 50%;
+  background: #ef4444;
+  animation: pulse 2s cubic-bezier(0.4, 0, 0.6, 1) infinite;
+}
+
 /* ── Content ── */
 .content {
   flex: 1;