feat: add utility functions with unit tests

Add formatCurrency, formatDate, and storeSlugs utilities in src/utils/
with 21 vitest unit tests covering standard and edge cases.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/utils/formatCurrency.ts

@@ -0,0 +1,10 @@
+export function formatCurrency(
+  cents: number,
+  locale = 'en-US',
+  currency = 'USD'
+): string {
+  return new Intl.NumberFormat(locale, {
+    style: 'currency',
+    currency,
+  }).format(cents / 100);
+}