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/formatDate.ts

@@ -0,0 +1,34 @@
+export function formatDate(
+  date: string | Date,
+  style: 'short' | 'long' | 'relative' = 'short'
+): string {
+  const d = typeof date === 'string' ? new Date(date) : date;
+
+  if (style === 'short') {
+    return d.toLocaleDateString('en-US', {
+      month: 'short',
+      day: 'numeric',
+      year: 'numeric',
+    });
+  }
+
+  if (style === 'long') {
+    return d.toLocaleDateString('en-US', {
+      weekday: 'long',
+      month: 'long',
+      day: 'numeric',
+      year: 'numeric',
+    });
+  }
+
+  // relative
+  const diff = Date.now() - d.getTime();
+  const seconds = Math.floor(diff / 1000);
+  if (seconds < 60) return 'just now';
+  const minutes = Math.floor(seconds / 60);
+  if (minutes < 60) return `${minutes}m ago`;
+  const hours = Math.floor(minutes / 60);
+  if (hours < 24) return `${hours}h ago`;
+  const days = Math.floor(hours / 24);
+  return `${days}d ago`;
+}