Get account balance

Return the current BRL balance for the company associated with the API key. The endpoint takes no parameters — the company is always derived from the API key. The three balance buckets are independent and additive: available (withdrawable now), frozen (reserved for in-flight withdrawals, refunds, or operational holds — not currently withdrawable), and held (confirmed but pending settlement). The total account balance is available + frozen + held. A sandbox API key always returns zeros, since sandbox transactions do not accumulate to a real balance.