`formatTokenUnits` and `formatCreditBalance` produce malformed output for negative BigInt values

[Sertug17]

In src/shared/utils.ts lines 40-55, both formatTokenUnits and formatCreditBalance produce malformed strings when given negative bigint inputs.

JavaScript BigInt division truncates toward zero and the remainder carries the sign of the dividend:

formatTokenUnits(-5n, 6)
// whole = 0n, fractional = -5n
// (-5n).toString() = "-5", padStart(6, "0") = "0000-5"
// Result: "0.0000-5"

formatTokenUnits(-1234567n, 6)
// whole = -1n, fractional = -234567n
// Result: "-1.-234567"

Current callers pass non-negative values (clamped via deposit > spent ? deposit - spent : 0n), so this is not triggered at runtime today, but the function signature accepts any bigint a trap for future callers.

Note: sibling function formatMicroUnits (line 31) is safe because its regex guard /^\d+$/ rejects negative strings.

Suggested fix:

export function formatTokenUnits(value: bigint, decimals: number) {
  const negative = value < 0n;
  const abs = negative ? -value : value;
  const divisor = 10n ** BigInt(decimals);
  const whole = abs / divisor;
  const fractional = abs % divisor;
  const formatted = decimals === 0
    ? whole.toString()
    : `${whole}.${fractional.toString().padStart(decimals, "0")}`;
  return negative ? `-${formatted}` : formatted;
}

Apply the same pattern to formatCreditBalance.