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>

backend/internal/modules/chat/repository.go

@@ -777,12 +777,13 @@ func (r *Repository) TransferConversation(principal Principal, conversationID ui
 func (r *Repository) GetAvailableSupportAdmins() ([]SupportAdminDTO, error) {
 	// 查询所有有 chat:view 权限且状态为 active 的管理员
 	type adminRow struct {
-		ID       uint64
-		Nickname string
+		ID            uint64
+		Nickname      string
+		SupportStatus string
 	}
 	var admins []adminRow
 	err := r.db.Table("admin_users AS au").
-		Select("DISTINCT au.id, COALESCE(NULLIF(au.nickname, ''), au.username) AS nickname").
+		Select("DISTINCT au.id, COALESCE(NULLIF(au.nickname, ''), au.username) AS nickname, au.support_status").
 		Joins("JOIN admin_user_roles AS aur ON aur.admin_user_id = au.id").
 		Joins("JOIN role_permissions AS rp ON rp.role_id = aur.role_id").
 		Joins("JOIN permissions AS p ON p.id = rp.permission_id").
@@ -817,9 +818,10 @@ func (r *Repository) GetAvailableSupportAdmins() ([]SupportAdminDTO, error) {
 	result := make([]SupportAdminDTO, len(admins))
 	for i, a := range admins {
 		result[i] = SupportAdminDTO{
-			ID:        a.ID,
-			Nickname:  a.Nickname,
-			ChatCount: loadMap[a.ID],
+			ID:            a.ID,
+			Nickname:      a.Nickname,
+			SupportStatus: a.SupportStatus,
+			ChatCount:     loadMap[a.ID],
 		}
 	}
 	return result, nil