Fix integer overflow in trade money calculation

[kwsantiago]

Summary

The trade confirmation handler computes new money balances using int32_t arithmetic:

plr->money = plr->money + plr2->moneyInTrade - plr->moneyInTrade;

Since money and moneyInTrade are both int32_t, the intermediate sum can overflow, allowing players to end up with negative or wrapped-around money values. A crafted trade can exploit this for money duplication.

What changed

• Compute new balances using int64_t before committing
• Validate that neither player is offering more money than they have
• Validate that resulting balances are within [0, INT32_MAX]
• Abort the trade cleanly if any check fails