test: add unit tests for formatUsd in lib/utils.ts

[Xaxxoo]

Summary

• Add comprehensive unit tests for the formatUsd() utility function in src/lib/utils.ts
• Tests cover: typical values, zero, negative numbers, NaN/undefined inputs, comma grouping, rounding behavior, and edge cases

Test plan

• Run npm test and verify all new tests pass
• Verify test coverage for formatUsd edge cases (NaN, undefined, negative zero)

Closes #130

Generated with Claude Code

[drips-wave]

@Xaxxoo Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits