feat: 添加客服在线状态和聊天粘贴图片功能

**客服在线状态：**
- 后端：添加 admin_users.support_status 字段（online/offline/busy）
- 接口：PUT /admin/me/support-status 更新状态
- 前端：管理员顶部导航显示状态切换器（仅客服权限可见）
- 转接对话框显示客服在线状态和智能排序（在线优先）

**聊天粘贴图片：**
- 用户端、移动端、管理端聊天输入框支持 Ctrl+V/Cmd+V 直接粘贴图片
- 自动调用图片压缩和上传逻辑，与文件选择上传保持一致
- 最多支持粘贴9张图片

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/layouts/AdminLayout.vue

@@ -20,13 +20,14 @@ import { ElMessage } from 'element-plus'
 import { computed, ref } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 
-import { logoutAdmin } from '@/features/admin'
+import { logoutAdmin, updateSupportStatus } from '@/features/admin'
 import { useAdminSessionStore } from '@/stores/adminSession'
 
 const router = useRouter()
 const route = useRoute()
 const adminSession = useAdminSessionStore()
 const isCollapsed = ref(false)
+const updatingStatus = ref(false)
 
 interface NavItem {
   label: string
@@ -60,6 +61,34 @@ const navItems = computed(() => {
 
 const activeMenu = computed(() => route.path)
 const adminName = computed(() => adminSession.nickname || adminSession.username || '管理员')
+const supportStatus = computed(() => adminSession.supportStatus || 'offline')
+const hasChatPermission = computed(() => adminSession.hasPermission('chat:view'))
+
+const statusLabels: Record<string, string> = {
+  online: '在线',
+  offline: '离线',
+  busy: '忙碌',
+}
+
+const statusColors: Record<string, string> = {
+  online: '#10b981',
+  offline: '#9ca3af',
+  busy: '#f59e0b',
+}
+
+async function handleStatusChange(status: 'online' | 'offline' | 'busy') {
+  if (updatingStatus.value) return
+  updatingStatus.value = true
+  try {
+    await updateSupportStatus(status)
+    adminSession.setSupportStatus(status)
+    ElMessage.success(`已切换为${statusLabels[status]}`)
+  } catch {
+    ElMessage.error('状态更新失败')
+  } finally {
+    updatingStatus.value = false
+  }
+}
 
 async function handleLogout() {
   try {
@@ -110,6 +139,28 @@ async function handleLogout() {
           </el-breadcrumb>
         </div>
         <div class="topbar-right">
+          <el-dropdown v-if="hasChatPermission" trigger="click" @command="handleStatusChange" :disabled="updatingStatus">
+            <span class="status-badge" :style="{ borderColor: statusColors[supportStatus] }">
+              <span class="status-dot" :style="{ backgroundColor: statusColors[supportStatus] }"></span>
+              <span class="status-text">{{ statusLabels[supportStatus] }}</span>
+            </span>
+            <template #dropdown>
+              <el-dropdown-menu>
+                <el-dropdown-item command="online">
+                  <span class="status-dot" style="background-color: #10b981;"></span>
+                  在线
+                </el-dropdown-item>
+                <el-dropdown-item command="busy">
+                  <span class="status-dot" style="background-color: #f59e0b;"></span>
+                  忙碌
+                </el-dropdown-item>
+                <el-dropdown-item command="offline">
+                  <span class="status-dot" style="background-color: #9ca3af;"></span>
+                  离线
+                </el-dropdown-item>
+              </el-dropdown-menu>
+            </template>
+          </el-dropdown>
           <el-dropdown trigger="click">
             <span class="user-info">
               <el-avatar :size="32" :icon="User" />
@@ -256,6 +307,39 @@ async function handleLogout() {
 .topbar-right {
   display: flex;
   align-items: center;
+  gap: 16px;
+}
+
+.status-badge {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 12px;
+  border: 1px solid;
+  border-radius: 16px;
+  cursor: pointer;
+  transition: all 0.2s;
+}
+
+.status-badge:hover {
+  background-color: #f5f7fa;
+}
+
+.status-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  display: inline-block;
+}
+
+.status-text {
+  font-size: 13px;
+  color: #606266;
+  font-weight: 500;
+}
+
+:deep(.el-dropdown-menu__item .status-dot) {
+  margin-right: 8px;
 }
 
 .user-info {