【界面UI】通用倒计时控件：活动、拍卖与刷新共用

---通用倒计时：终点时间戳驱动
Countdown.pool = {}

function Countdown.bind(node, key, endAt, onDone)
    local txt = GUI:Text_Create(node, "cd_" .. key, 0, 0,
        14, "#FFD700", "--:--")
    Countdown.pool[key] = {
        node = txt, endAt = endAt, onDone = onDone,
    }
    Countdown.refresh(key)
end

---统一刷新：每秒一轮
SL:Schedule(1.0, function()
    for key, item in pairs(Countdown.pool) do
        Countdown.refresh(key)
    end
end)

function Countdown.refresh(key)
    local item = Countdown.pool[key]
    if not item then return end
    local left = item.endAt - os.time()
    if left <= 0 then
        GUI:Text_setString(item.node, "已结束")
        if item.onDone then item.onDone(key) end
        Countdown.pool[key] = nil
    else
        GUI:Text_setString(item.node, SL:SecondToHMS(left))
    end
end

---活动面板：开放倒计时
function ActivityPanel.show(info)
    Countdown.bind(ActivityPanel.node, "act_" .. info.id,
        info.openAt, function(key)
            SL:ShowSystemTips("活动已开启")
            ActivityPanel.reload()
        end)
end

---拍卖行：截拍倒计时带紧急变色
function Auction.bid(item)
    local left = item.endAt - os.time()
    if left <= 30 then
        GUI:Text_setColor(Countdown.pool[item.id].node, "#FF4444")
    end
    SL:SendNetMsg(MSGID.AUCTION_BID, item.id, item.price, 0, "")
end