Show negative amounts with a minus sign

[sergical]

formatUsd now puts a minus sign in front of the dollar sign for negative amounts, so refunds read as -$5 instead of $-5.

Created with Claude Code

[github-actions]

Verdict: Needs work — the fix for negative values is correct but the function still produces incorrectly-formatted currency strings (pre-existing bug, not addressed).

Correctness

• No related automated tests exist (run_tests found 0 tests), so this change has no test coverage.
• Core pre-existing bug remains unfixed: the function never rounds/pads to two decimal places. formatUsd(100) → "$1" instead of "$1.00"; formatUsd(150) → "$1.5" instead of "$1.50"; formatUsd(5) → "$0.05" works by luck, but formatUsd(1005) can produce floating-point artifacts. This directly undermines the purpose of a currency formatter and should be fixed in the same PR (e.g., via .toFixed(2) or Intl.NumberFormat).
• No thousands separators for large amounts (e.g. "$1234567.89" instead of "$1,234,567.89").
• Unhandled inputs: NaN → "$NaN", Infinity/-Infinity → "$Infinity"/-$Infinity". Minor, but worth a guard or documented assumption.
• The new sign logic itself is correct: Math.abs(cents) < 0 short-circuits properly, -0 is handled sensibly (prints "$0"), and negative values now display with a leading - rather than the old buggy behavior (e.g., old code gave "$-1" style results from -100/100). This part of the diff is a genuine, well-targeted improvement.

Style

• Prefer a template literal over + concatenation for readability: `${sign}$${Math.abs(cents) / 100}`.
• Since proper decimal formatting is needed anyway (see correctness above), consider replacing the whole function body with Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(cents / 100), which would also subsume the new sign-handling logic and be more maintainable.
• sign is an adequate name; not a blocker.

Summary

The negative-number sign fix is correct and well-scoped, but the PR leaves significant pre-existing formatting correctness gaps (no decimal padding/rounding, no grouping) that make the function unreliable as a currency formatter. Recommend addressing the decimal-formatting issue before merging, or explicitly scoping this PR to "sign only" and filing a follow-up for formatting precision.