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/storeSlugs.ts

@@ -0,0 +1,13 @@
+export const STORE_SLUGS: Record<string, { name: string; color: string; icon: string }> = {
+  meijer: { name: 'Meijer', color: '#e31837', icon: '/icons/stores/meijer.svg' },
+  kroger: { name: 'Kroger', color: '#0033a0', icon: '/icons/stores/kroger.svg' },
+  target: { name: 'Target', color: '#cc0000', icon: '/icons/stores/target.svg' },
+};
+
+export function getStore(slug: string) {
+  return STORE_SLUGS[slug.toLowerCase()] ?? null;
+}
+
+export function getStoreName(slug: string): string {
+  return getStore(slug)?.name ?? slug;
+}