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/features/admin/components/TransferDialog.vue

@@ -20,11 +20,37 @@ const submitting = ref(false)
 const admins = ref<SupportAdmin[]>([])
 const selectedAdminId = ref<number | null>(null)
 
-// 按会话数排序，空闲客服优先
+// 按在线状态和会话数排序：在线客服优先，会话数少的优先
 const sortedAdmins = computed(() => {
-  return [...admins.value].sort((a, b) => a.chat_count - b.chat_count)
+  return [...admins.value].sort((a, b) => {
+    // 在线状态优先级：online > busy > offline
+    const statusPriority: Record<string, number> = { online: 0, busy: 1, offline: 2 }
+    const aPriority = statusPriority[a.support_status] ?? 2
+    const bPriority = statusPriority[b.support_status] ?? 2
+    if (aPriority !== bPriority) return aPriority - bPriority
+    // 同状态下按会话数排序
+    return a.chat_count - b.chat_count
+  })
 })
 
+function getSupportStatusLabel(status: string) {
+  const labels: Record<string, string> = {
+    online: '在线',
+    offline: '离线',
+    busy: '忙碌',
+  }
+  return labels[status] || '未知'
+}
+
+function getSupportStatusColor(status: string) {
+  const colors: Record<string, string> = {
+    online: '#10b981',
+    offline: '#9ca3af',
+    busy: '#f59e0b',
+  }
+  return colors[status] || '#9ca3af'
+}
+
 watch(() => props.modelValue, (val) => {
   visible.value = val
   if (val) {
@@ -84,16 +110,19 @@ function getStatusTag(count: number) {
           class="admin-item"
         >
           <div class="admin-info">
-            <el-avatar :size="36" :icon="User" />
+            <div class="admin-avatar-wrap">
+              <el-avatar :size="36" :icon="User" />
+              <span class="status-indicator" :style="{ backgroundColor: getSupportStatusColor(admin.support_status) }"></span>
+            </div>
             <div class="admin-detail">
               <span class="admin-name">{{ admin.nickname }}</span>
               <span class="admin-id">ID: {{ admin.id }}</span>
             </div>
           </div>
           <div class="admin-status">
-            <el-tag :type="getStatusTag(admin.chat_count).type" size="small">
-              {{ getStatusTag(admin.chat_count).text }}
-            </el-tag>
+            <span class="support-status" :style="{ color: getSupportStatusColor(admin.support_status) }">
+              {{ getSupportStatusLabel(admin.support_status) }}
+            </span>
             <span class="admin-count">{{ admin.chat_count }} 个会话</span>
           </div>
         </el-radio>
@@ -159,6 +188,21 @@ function getStatusTag(count: number) {
   min-width: 0;
 }
 
+.admin-avatar-wrap {
+  position: relative;
+  flex-shrink: 0;
+}
+
+.status-indicator {
+  position: absolute;
+  right: 0;
+  bottom: 0;
+  width: 10px;
+  height: 10px;
+  border-radius: 50%;
+  border: 2px solid #fff;
+}
+
 .admin-detail {
   display: flex;
   flex-direction: column;
@@ -188,6 +232,11 @@ function getStatusTag(count: number) {
   flex-shrink: 0;
 }
 
+.support-status {
+  font-size: 13px;
+  font-weight: 600;
+}
+
 .admin-count {
   color: #9ca3af;
   font-size: 12px;