CVE-2026-34064: nimiq-account: Vesting insufficient funds error can panic

VestingContract::can_change_balance returns AccountError::InsufficientFunds when new_balance < min_cap, but it constructs the error using balance: self.balance - min_cap. Coin::sub panics on underflow, so if an attacker can reach a state where min_cap > balance, the node crashes while trying to return an error.

The min_cap > balance precondition is attacker-reachable because the vesting contract creation data (32-byte format) allows encoding total_amount without validating total_amount <= transaction.value (the real contract balance). After creating such a vesting contract, the attacker can broadcast an outgoing transaction to trigger the panic during mempool admission and block processing.