feat: add commas to currency formatting

[cnaples79]

Summary

• format currency output with thousands separators while preserving sign
• add unit tests covering positive and negative amounts

Rationale

• aligns displayed numeric values with expected comma-separated currency formatting (issue Add commas to formatting #3)

Changes

• update FormatCurrency to insert commas and keep the sign separate
• add formatter tests for comma formatting on positive/negative values

Fixes #3

[tryonlinux]

Thanks for the PR! However it looks like we both were working on the comma thing (#4 ) at almost the same time and I already merged the comma fix in about 20 mins before you created this PR! Sorry for the duplicative work! Didn't expect the issue to be picked up so quickly by someone else.