Add unit tests for utils and module pure functions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/module/numberLabel.test.js

@@ -0,0 +1,26 @@
+import { describe, it, expect } from 'vitest';
+import numberLabel from '@/module/numberLabel.js';
+
+describe('numberLabel', () => {
+  it('formats small numbers without commas', () => {
+    expect(numberLabel(1)).toBe('1');
+    expect(numberLabel(999)).toBe('999');
+  });
+
+  it('formats thousands with commas', () => {
+    expect(numberLabel(1000)).toBe('1,000');
+    expect(numberLabel(12345)).toBe('12,345');
+  });
+
+  it('formats millions with commas', () => {
+    expect(numberLabel(1234567)).toBe('1,234,567');
+  });
+
+  it('preserves decimal places', () => {
+    expect(numberLabel(1234.56)).toBe('1,234.56');
+  });
+
+  it('handles zero', () => {
+    expect(numberLabel(0)).toBe('0');
+  });
+});