Fix cash shop lock after purchase failure

[Copilot]

Failed cash shop operations (buy item, buy package, increase slots) left the client UI locked. The client waits for a SendChannelCSAction response to complete the operation flow, but several error paths only sent SendChannelCSUpdateAmounts, which updates the NX/MP display without signaling operation completion.

Changes

• BuyItem: Replace packetCashShopUpdateAmounts with packetCashShopError for unknown SN, not-on-sale, invalid currency, and item creation failure paths. Add missing error response for unreachable currency branch at charge time.
• BuyPackage: Same treatment for unknown SN, empty package, invalid currency, zero itemID, item creation failure, and GiveItem failure paths.
• IncreaseSlots: Add missing error response for unknown currency type (previously a bare return).

Before:

if !ok || commodity.ItemID == 0 {
    plr.Send(packetCashShopUpdateAmounts(plrNX, plrMaplePoints)) // client stays locked
    return
}

After:

if !ok || commodity.ItemID == 0 {
    plr.Send(packetCashShopError(opcode.SendCashShopBuyFailed, constant.CashShopErrorOutOfStock))
    return
}