实现 P0 自动分配、离线留言与可嵌入 Widget SDK

- 会话创建时按负载自动分配在线客服，无客服则进入离线模式
- 新增 /api/widget/leave-message 沉淀联系方式与留言事件
- 访客端离线留言表单；工作台展示离线留言/自动分配事件
- 提供 public/widget.js + /widget/embed 嵌入方案

server/cmd/seed/main.go

@@ -68,7 +68,7 @@ func seed() {
 
 	// Channels for Tenant 1
 	channels := []model.Channel{
-		{TenantID: tenants[0].ID, Type: "web", Name: "网页聊天", Status: "enabled", ScriptCode: `<script src="https://cs.example.com/widget.js" data-id="WK_8a3f2e"></script>`},
+		{TenantID: tenants[0].ID, Type: "web", Name: "网页聊天", Status: "enabled", ScriptCode: `<script src="/widget.js" data-id="WK_8a3f2e"></script>`},
 		{TenantID: tenants[0].ID, Type: "wechat", Name: "微信公众号", Status: "enabled"},
 		{TenantID: tenants[0].ID, Type: "app", Name: "APP内嵌", Status: "disabled"},
 	}