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/__tests__/storeSlugs.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest';
+import { getStore, getStoreName, STORE_SLUGS } from '../storeSlugs';
+
+describe('storeSlugs', () => {
+  describe('STORE_SLUGS constant', () => {
+    it('contains meijer, kroger, and target', () => {
+      expect(STORE_SLUGS).toHaveProperty('meijer');
+      expect(STORE_SLUGS).toHaveProperty('kroger');
+      expect(STORE_SLUGS).toHaveProperty('target');
+    });
+  });
+
+  describe('getStore', () => {
+    it('returns store data for known slug', () => {
+      const store = getStore('meijer');
+      expect(store).toEqual({
+        name: 'Meijer',
+        color: '#e31837',
+        icon: '/icons/stores/meijer.svg',
+      });
+    });
+
+    it('returns null for unknown slug', () => {
+      expect(getStore('unknown-store')).toBeNull();
+    });
+
+    it('is case insensitive', () => {
+      expect(getStore('KROGER')).toBeTruthy();
+      expect(getStore('Target')).toBeTruthy();
+    });
+  });
+
+  describe('getStoreName', () => {
+    it('returns store name for known slug', () => {
+      expect(getStoreName('kroger')).toBe('Kroger');
+    });
+
+    it('returns raw slug for unknown store', () => {
+      expect(getStoreName('unknown-store')).toBe('unknown-store');
+    });
+
+    it('is case insensitive', () => {
+      expect(getStoreName('TARGET')).toBe('Target');
+    });
+  });
+});