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/stores/adminSession.ts

@@ -10,6 +10,7 @@ export const useAdminSessionStore = defineStore('adminSession', {
     adminId: Number(localStorage.getItem('admin_id') || 0),
     username: localStorage.getItem('admin_username') || '',
     nickname: '',
+    supportStatus: (localStorage.getItem('admin_support_status') || 'offline') as 'online' | 'offline' | 'busy',
     roles: [] as AdminRole[],
     permissions: [] as string[],
   }),
@@ -41,15 +42,18 @@ export const useAdminSessionStore = defineStore('adminSession', {
       this.adminId = 0
       this.username = ''
       this.nickname = ''
+      this.supportStatus = 'offline'
       this.roles = []
       this.permissions = []
       clearAuthStorage('admin')
+      localStorage.removeItem('admin_support_status')
     },
     syncFromStorage() {
       this.token = getAccessToken('admin')
       this.refreshToken = getRefreshToken('admin')
       this.adminId = Number(localStorage.getItem('admin_id') || 0)
       this.username = localStorage.getItem('admin_username') || ''
+      this.supportStatus = (localStorage.getItem('admin_support_status') || 'offline') as 'online' | 'offline' | 'busy'
     },
     applySession(admin: AdminUser, accessToken: string, refreshToken: string) {
       this.token = accessToken
@@ -64,10 +68,16 @@ export const useAdminSessionStore = defineStore('adminSession', {
       this.adminId = admin.id
       this.username = admin.username
       this.nickname = admin.nickname
+      this.supportStatus = admin.support_status || 'offline'
       this.roles = admin.roles || []
       this.permissions = admin.permissions || []
       localStorage.setItem('admin_id', String(admin.id))
       localStorage.setItem('admin_username', admin.username)
+      localStorage.setItem('admin_support_status', admin.support_status || 'offline')
+    },
+    setSupportStatus(status: 'online' | 'offline' | 'busy') {
+      this.supportStatus = status
+      localStorage.setItem('admin_support_status', status)
     },
   },
 })