feat: 新增管理员踢下线 VPN 客户端功能

- vpn/service.go: ClientInfo 增加 user_id 字段，新增 KickUser 方法
  用读锁收集目标连接后逐个关闭 WebSocket，返回断开数量
- vpn/tunnel.go: info() 填充 UserID
- handler/vpn.go: 新增 KickUserClient handler，校验用户存在后调用 KickUser
- router.go: 注册 DELETE /api/admin/vpn/clients/:id
- VpnView.vue: 在线客户端表格新增操作列与踢下线按钮，handleKick 确认后调用 API 并刷新
- zh.ts/en.ts: 新增 kick/confirmKick/kickFailed 国际化 key

internal/router/router.go

@@ -46,6 +46,7 @@ func Setup(r *gin.Engine) {
 		admin.GET("/vpn/reservations", handler.ListVpnReservations)
 		admin.POST("/vpn/reservations", handler.CreateVpnReservation)
 		admin.DELETE("/vpn/reservations/:id", handler.DeleteVpnReservation)
+		admin.DELETE("/vpn/clients/:id", handler.KickUserClient)
 	}
 
 	distDir := http.Dir("./dist")