Add PaycheckView unit tests

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>

client/src/pages/PaycheckView.jsx

@@ -25,6 +25,8 @@ function todayISO() {
   return new Date().toISOString().slice(0, 10);
 }
 
+export { ordinal, formatCurrency, formatPayDate };
+
 // ─── PaycheckColumn ───────────────────────────────────────────────────────────
 
 function PaycheckColumn({ paycheck, onBillPaidToggle, categories, onOtePaidToggle, onOteDelete, onOteAdd, onGenerate, onAmountSave, onBillAmountSave, onFinancingPaidToggle }) {