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__/formatDate.test.ts

@@ -0,0 +1,62 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { formatDate } from '../formatDate';
+
+describe('formatDate', () => {
+  describe('short style', () => {
+    it('formats an ISO date string', () => {
+      const result = formatDate('2024-03-15', 'short');
+      expect(result).toMatch(/Mar 15, 2024/);
+    });
+
+    it('formats a Date object', () => {
+      const result = formatDate(new Date('2024-03-15'), 'short');
+      expect(result).toMatch(/Mar 15, 2024/);
+    });
+  });
+
+  describe('long style', () => {
+    it('formats with weekday and full month name', () => {
+      const result = formatDate('2024-03-15', 'long');
+      expect(result).toMatch(/Friday/);
+      expect(result).toMatch(/March/);
+    });
+  });
+
+  describe('relative style', () => {
+    beforeEach(() => {
+      vi.useFakeTimers();
+    });
+
+    afterEach(() => {
+      vi.useRealTimers();
+    });
+
+    it('returns "just now" for very recent dates', () => {
+      const now = new Date('2024-01-01T12:00:00Z');
+      vi.setSystemTime(now);
+      const result = formatDate(new Date('2024-01-01T11:59:59Z'), 'relative');
+      expect(result).toBe('just now');
+    });
+
+    it('returns minutes ago', () => {
+      const now = new Date('2024-01-01T12:00:00Z');
+      vi.setSystemTime(now);
+      const result = formatDate(new Date('2024-01-01T11:45:00Z'), 'relative');
+      expect(result).toBe('15m ago');
+    });
+
+    it('returns hours ago', () => {
+      const now = new Date('2024-01-01T12:00:00Z');
+      vi.setSystemTime(now);
+      const result = formatDate(new Date('2024-01-01T09:00:00Z'), 'relative');
+      expect(result).toBe('3h ago');
+    });
+
+    it('returns days ago', () => {
+      const now = new Date('2024-01-05T12:00:00Z');
+      vi.setSystemTime(now);
+      const result = formatDate(new Date('2024-01-01T12:00:00Z'), 'relative');
+      expect(result).toBe('4d ago');
+    });
+  });
+});