Export ordinal/formatCurrency/formatPayDate for direct testing. Tests cover
pure function formatting, virtual/saved rendering, preview banner, error state,
month navigation (including year wrap), bill display, remaining balance, and
gross/net amounts. Uses vi.useFakeTimers({ toFake: ['Date'] }) to control
current date without faking setTimeout (which would break RTL's waitFor).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
